All rules are taken from the May 2010 Errata versions. Major disparities with the DMG will be noted.

Saddles: Ignore this rule, it was errataed out.

Size: Ignore the 'no smaller than large' part, also errataed out.

Space: The rider and mount both occupy the mount’s space. However, the origin squares of the rider’s powers and other effects do not change to the mount’s size. Whenever the rider uses an effect that has an origin square (such as a melee, a ranged, an area, or a close power), the rider first picks where that square is located in the mount’s space, and the effect uses that origin square. For example, if a Medium rider uses a close burst attack power, the rider chooses a single square within the mount’s space, and the burst emanates from that square. This rule means that if the burst targets each creature within it, rather than each enemy, it can hit the mount. (Please note that when using Blast powers, you can choose an edge square of your mount, and thus never have to include it the effect. Also – this rule interacts a little oddly with effects that state 'within x squares of you', for example the Bard's Virtue class features. Because you share the mount's space, which is larger than your's, the range of these effects increases. This rule also does not specify whether you count as adjacent to your mount when on it. Personally, I say you do, as it makes sense logically and mechanically, but this is open to DM interpretation. Where it is relevant, I will be assuming that you do count.) Also: Damon_Tor put rather nicely in the Rules Q&A forum:

Targeting the Mount and Rider: Even though the mount and rider occupy the same space, they are still separate creatures and are targeted separately. For example, an attack that targets only one creature can target either the mount or the rider, not both. In contrast, area and burst attacks can affect both mount and rider, since the two are in the same space.

Mount Benefits: Many mounts offer special attacks or benefits they can use or grant to their riders. These abilities range from flat bonuses, such as an AC bonus to the rider, to special attacks that the mount can use. The Monster Manual details the benefits that many creatures grant if the rider meets a minimum level and has the Mounted Combat feat. If the rider doesn’t meet a mount’s prerequisites, he or she can ride it but doesn’t gain the mount’s special benefits.

Provoking Opportunity Attacks: If the mount’s movement provokes an opportunity attack, the attacker chooses to target either the mount or the rider, since the two of them move together. However, if the mount or the rider provokes an opportunity attack by using a ranged or an area power, the attacker must target whichever one of them provoked the opportunity attack.

Forced Movement: If the mount is pulled, pushed, or slid, the rider moves with it. If the rider is pulled, pushed, or slid and the mount isn’t, the rider can have the two of them move together. Otherwise, the rider is dismounted and falls prone in the destination space of the forced movement.

Teleportation: If either the mount or the rider is teleported, the other does not teleport with it. If the mount is teleported without the rider, the rider is dismounted and falls prone. (There is an item, the Saddle of the Nightmare, that allows you to teleport with your mount. Please see it's section below for details.)

Mounting and Dismounting:

• Mount (Move Action): The rider mounts a creature adjacent to it and enters its space.
• Dismount (Move Action): The rider dismounts, moving from the mount’s space to a square adjacent to it.

 

(DMG lists these as Standard actions)

Initiative: The mount and rider act on the rider’s initiative count, even if the mount had a different initiative before the rider mounted it. The two continue to act on the same initiative count after the rider dismounts. A monster and its mount have separate turns, whereas an adventurer and his or her mount have a single turn.

Actions: An adventurer and his or her mount have one combined set of actions: a standard action, a move action, and a minor action. The player chooses how the two creatures use the actions on the adventurer’s turn. The adventurer and the mount also share a single immediate action each round and a single opportunity action each turn. If the adventurer dismounts, the two still share one set of actions on that turn, but have separate sets of actions thereafter.

Mount Attacks: (Replaces the poorly-named Attacking Mounts passage) The mount takes a –2 penalty to attack rolls unless the rider has the Mounted Combat feat. (The errataed version of this rule makes no mention of the previous ability to use standard actions to use the mount's attacks instead of your own. Given that the -2 penalty still applies, it can be assumed that this is still allowed, and I will be rating mounts on the assumption of being able to do so.)

Charging: When the rider charges, the rider and mount move up to the mount’s speed and then the rider makes the charge attack. When the mount charges, it follows the normal rules for a charge. (The DMG specifies that either the rider or the mount may make the charge attack.)

Squeezing: When the mount squeezes, the rider is also considered to be squeezing. (And takes the relevant penalties.)

Falling Prone: If the mount falls prone, the rider is dismounted and falls prone in an unoccupied space of the rider’s choice adjacent to the now-prone mount. However, if the mount is flying when it falls prone, it instead falls and the rider isn’t dismounted unless the mount lands and falls prone itself. A rider who is knocked prone can immediately make a saving throw. On a roll lower than 10, the rider is dismounted and falls prone in an unoccupied space of the rider’s choice adjacent to the mount. On a roll of 10 or higher, the rider is neither dismounted nor knocked prone. A rider who voluntarily drops prone falls prone in an unoccupied space of the rider’s choice adjacent to the mount.

Mounts and Items: (AV pg 122) A creature serving as your companion or mount has a single magic item slot that you can activate using your own actions (and not the creature's). A companion that doubles as a mount can use a mount slot item in place of a companion item, and vice versa. (This does not allow a mount to use companion items, these are specific to the ranger beast companion. The same goes for familiar slot items. Also, Barding items, while listed seperately in the CB and the Compendium, still use up this one item slot. I have seen it played where they don't, citing AV pg 14, which describes barding as 'Armor for your mount', however this is a slight bend in the rules when using magical barding.)

Is your, or one of your party members' mounts your ally? This is important for effects that rely on adjacent allies, allies in burst powers, etc. The answer is yes, yes they are. See here for confirmation from Greg Bilsland. Even without that though, it is logical that a mount willingly serving you is your ally.

Presented in the order listed in the CB.

Blade Spider: (MM, lvl 10) Tremorsense 10 and Spider Climb 6 give this creature some versatility, and it's Mount ability "Combined Attack" grants an encounter power (errataed from at-will in the MM) for a free action attack when you make a melee attack to cause 10 ongoing poison dmg (save ends). "Double Attack" at-will, and it's MBA causes weakened and ongoing 5 poison dmg (save ends both). Useful for the poison themed character, or those regularly exploring the underdark.

Camel: (AV, lvl 1) It's a camel. WYSIWYG. No mount action, but it's a lvl 1 mount, and very easy on the wallet at 75gp.

Celestial Charger: (MM, lvl 10 Soldier) The archetypal Paladin's mount. Speed 8, and it's Zephyr Footing ability allows to ignore difficult terrain and move across any solid or liquid surface. Very useful. It's Mount Action "Celestial Charge" causes it's rider to deal an extra 2d6 radiant dmg on charge attacks. It's listed as an Encounter power in the compendium (at-will in the MM), but worded as a passive effect, so you may want to discuss the usage of this one with your DM. VERY powerful if it is passive, as all the usual comments for charge and radiant support apply here. +5 to saves vs fear is a nice addon. Lawful Good alignment, so be nice to it!

Dire Boar: (MM, lvl 6) Quite the mount for a charger character. Speed 8, for starters. It's "Rabid Charger" trait allows it to make it's Gore attack in addition to your charge attack, which is then boosted by it's "Furious Charge" trait – Gore, when charging, does an extra 5 dmg, pushes 2 squares and knocks prone. Gore itself also deals extra dmg to a prone target.

Dire Shark: (AV, lvl 14) Obviously your mileage (no pun intended) with this mount will vary by how much underwater or seaborne combat you see. Swim speed 11, and it's Huge (yes, 3×3). It's passive trait "Waterborn" gives it an untyped +2 to attack when in water against creatures lacking a swim speed, which it also grants to it's rider with "Deft Swimmer" along with a +2 untyped AC against OAs. Very campaign dependant.

Dire Wolf: (MM, lvl 5) Here doggy! It's unimaginatively named "Combat Advantage" trait gives the Dire Wolf CA against a target that has one or more of the wolf's allies next to it. You count. The Dire Wolf knocks prone when hitting with CA, giving you an at-will prone if you're willing to spend your standard actions on your mount's attacks, but there are other mounts capable of this. It's "Pack Hunter" Mount action is of little use to a melee character who will most likely be flanking anyway, but for a ranged character, or someone using a Reach weapon, this is a better version of Distant Advantage – no limit on attack type and only requires one ally adjacent, not two in flanking. For Beastmaster Archer rangers, your pet counts. If you're hurting a a bit for feats, but want Distant Advantage, 1000gp for a Dire Wolf may prove very useful.

Elephant: (AV, lvl 8) Huge, an MBA with reach 2 and an at-will proning attack (Stamp) make this mount a solid choice for area-denial on the battlefield. "Trampling Charge" allows you to charge through one Medium or smaller opponent and make a Stamp attack against it in addition to the attack you make at the end of the charge.

Giant Ant: (AV, lvl 4) MBA knocks prone, and shift 2 squares instead of one while mounted. Useful for characters that need to easily reposition themselves, or are using Reach weapons.

Giant Lizard, Draft: (AV, lvl 4) Just use the Riding version instead, it's better in every way. That said, this is one of the cheaper mounts, and suited to certain terrain types for theme reasons.

Giant Lizard, Riding: (AV, lvl 6) Speed 9 (swamp walk) and climb 4 are useful movement types, but the real benefit of this mount is "Combined Attack" which allows the lizard to make a Claw attack against a target you make a melee attack against once per encounter. (Errataed from at-will in the AV)

Griffon: (MM, lvl 7) Fly 10, overland flight 15, and that's just the beginning. "Rabid Charger" allows the griffon to make two claw attacks in addition to your attack when charging, and is at-will, which are made at a +4 bonus instead of a +1 if flying thanks to "Thunder Charge" (Does not apply to your charge attack). If the mount becomes bloodied, it becomes even more powerful, gaining fear immunity, +2 untyped to attacks, -2 to all defences, and an extra move action per turn (!). Whether it is allowed this extra move action while ridden due to shared action rules is up to your DM. A very good choice for a charger character looking for a flying mount.

Hippogriff: (MM, lvl 5) Fly 10, overland flight 15 are good, but mind the speed 4 if grounded. At-will charge to prone while flying and "Flyby Attack" could come in useful, but "Aerial Agility" is probably it's best feature, an untyped +1 to all it's rider's defences while flying. If this is the only feature of this creature you like, the Wyvern at lvl 10 is probably a much better option.

Hippogriff Dreadmount: (MM, lvl 5 Soldier) As with the Hippogriff, mind the ground speed of 4. A good choice for a Defender, especially the Paladin who doesn't need his Immediates as much, with "Wing Slam", an at-will interrupt to knock prone someone who shifts or moves away from you. "Sturdy Mount" reduces all forced movements from attacks by 1, and allows it to immediately make saves against either you or it being knocked prone. Great for stickiness and staying in the battle.

Manticore: (MM lvl 10) It's Elite, and thus has an Action Point. Please see the rules section above regarding this. Fly 8, overland 10, decent. It's mount power is no benefit to you directly, but grants the manticore +2 to attack with it's range and area attacks. "Spike", it's ranged at-will does allow it to shift 3 after the attack, hit or miss, taking you with it, so it could be handy for sudden repositioning. It also has an at-will to perfom one melee and one ranged attack, with a shift of 1 square in between them. Overall, not really that useful to players, but being Chaotic Evil and the different mechanics for monster mounts make it a nice choice for a BBEG.

Nightmare: (MM, lvl 13) Evil! That aside, this mount has several useful features. Speed 10, Teleport 10, +2 AC vs OAs, Darkvision and resist 20 fire. "Hell's Ride" grants you resist 20 fire as well. It's MBA causes ongoing 5 fire (save ends) and "Hooves of Hell" (recharge 5,6) lets you set up a 10 square area of any shape that causes 10 fire auto-damage to any creature entering it. I'm sure you can find a use for it.

Rage Drake: (MM, lvl 5) "Raking Charge" is another vaguearity. Is it a passive benefit, or is it a standard action to charge and make two attacks? Given the wording on the Griffon, I believe to be the latter, which is somewhat less useful. "Raging Mount" is useful for melee, +2 untyped bonus to attack and damage, but you only get it while the drake is bloodied. "Bloodied Rage" increases the power of the mount's attacks while bloodied, but you're more likely to be making your own with the bonuses given, as the drake's attacks are nothing too special.

Rhinoceros: (AV, lvl 7 Soldier) It's mount action allows it tack it's MBA on to your charges, which is nice, but the Dire Boar does it better thanks to additional traits and being speed 8 to the Rhino's speed 6.

Riding Horse: (MM, lvl 1) A horse. As with the camel, no mount action, but lvl 1 and cheap.

Riding Shark: (AV, lvl 8) A lower level version of the Dire Shark which is large instead of huge. Otherwise, all the same comments apply.

Rimefire Griffon: (MM, lvl 20) Someone has expensive taste! Fly 10, Overland 15, and speed 5 (As opposed to the speed 4 of the hippogriffs) are decent. Resist 10 cold, 10 fire, which it also gives you with it's mount power. It's blast attack is decent, but only usable after hitting twice with it's bite attack, which means you're not gonna be able to use it often, unless you make a lot of OAs with the bite, or something similar. Flyby Attack can be useful, but overall, this mount asks a bit more of you than it gives in return. Best only as a source of resistance. Not worth it you have to sink the GP into it.

Sea Horse: (AV, lvl 5) If having to use an underwater mount, this is probably going to be the charger's choice. Swim 10 and a reach 2 MBA are nice, and it has the same "Waterborn" quality as the riding/dire shark, but it's "Aquatic Charge" rider power is better from an offensive standpoint. The same +2 to attack against non-swimmers, and your charge attack deals an extra 1d10 dmg. Still campaign dependant, and some may feel threatened in their manhood at the idea of riding one.

Skeletal Horse: (AV, lvl 11) Speed 10 and being undead, it gets resist 20 necrotic, which it grants to it's rider through "Shadow Symbiosis", and poison immunity, which it doesn't. Very useful, and probably readily available in an undead-heavy campaign.

Trihorn Behemoth: (AV, lvl 12 Soldier) Being huge is probably the only thing going for this mount. Speed 6 leaves a bit to be desired, and it's mount action "Protective Crest" grants a +1 shield bonus to AC and Reflex. There are mounts with better defense bonuses, better movement modes and generally better options. Avoid this one.

Warhorse: (MM, lvl 3) Speed 8, and "Trample" allows it to use a standard action to move through enemies and make a Reflex-targeting prone attack against them. +5 dmg bonus to your charge attacks is decent enough. You can do better for boosting charge capabilities, but it's a good option for it's price. If you do want a warhorse for role-play or theme reasons, consider the Magebred Destrier instead (Listed below, EPG section).

Wyvern: (MM, lvl 10) Of all the mounts in the CB, this one is probably the best option for a ranged character. Fly 8 (hover) lets you happily sit out of range and fire away, although do be wary of the speed 4 if you end up grounded. 2 Reach 2 attacks, one of which is an MBA, the other allowing a secondary attack for ongoing 10 poison, but the best effect is "Aerial Agility +2" granting it's rider an untyped +2 to all defences while flying. Definately a solid option, although if you're of the level where it's an option, give serious consideration to the Behir Stormsteed listed below (MM2 section) instead. Do also consider the Snaketongue Wyvern from the DMG2, which is a better version of the Wyvern.
 

Behir Stormsteed: (MM2, lvl 24 Soldier) (Not listed as a mount in the MM, errated to correct this) Oh my. Probably the best mount option for a Storm Sorceror or other Lightning themed character, if not one of the best mounts in the game. Speed 8, Fly 8 (Hover), Huge, Resist 15 Lightning, and an enormously powerful mount action. "Ride The Lightning" grants it's rider an untyped +2 to attack with Lightning powers, and 'The behir stormsteed can use its bite or lightning breath once per turn as a minor action.' Bite is a Reach 2 MBA that does additional Lightning damage, plus 5 auto Lightning damage to all creatures within 3 squares (Not party-friendly, but the Behir will be unaffected, and thanks to the rules on rider and mount spaces, you can make yourself be unaffected as well). Lightning Breath is a 2 target, reflex-targeting recharge 4,5,6 attack that slows on a hit and does not provoke OAs due to it's close burst 3 range (limited range, but effective none-the-less). Minor action attacks are a boon to any character, especially those using a Ring of Free Time. A mount worthy of the Epic Tier.

Nyfellar Mammoth: (MM2, lvl 17) Huge, Resist 10 cold, speed 8 (ice walk) and an at-will prone attack (That does cold damage for you Frostcheese users) are a good start, "Icebound Footing" reduces forced movement by 2 and lets the mammoth save vs being knocked prone (Does not benefit you like the Hippogriff Dreadmount). "Tusk Toss" is interesting, a recharge 4,5,6 reach 2 attack that slides 5 squares and causes falling damage from the slide. "Blizzard Trample"s recharge conditions make it a little infrequent to use, but resisted cold damage is still cold damage, even if it's zero. "Bitterwind Charge" interacts badly with the new version of the mount charge rules, which do not allow a mount to make an attack on a charge you initiate. If taken as allowing to make an attack on a charge in addition to your's, then it's useful, especially when it allows the use of "Tusk Toss" and "Blizzard Trample" – the latter of which technically allows you to charge 8, then move another 8 through enemies, Stamping each one, and as worded, you make a melee basic as a free action after each stamp, which does not have to target the same enemy! A powerful and somewhat abusable effect with a bit of an iffy wording, so you may want to work out with your DM how this should be used.

Rime Hound: (MM2, lvl 17) (Evil alignment) Huge, Elite (AP), Speed 8 (ice walk), Resist 20 cold, and a Reach 2 MBA that does Cold damage, and extra against a prone target. "Ice Spikes", it's mount action, lets you counter-attack when an enemy misses either of you with a melee attack, but does use up your immediate action. Possibly useful to a catch-22 designed defender. "Frost Storm" has controller-esque applications, but the party unfriendly nature of the difficult terrain created (The Rime Hound's ice walk allows it to ignore it) will limit it's usefulness, especially considering a burst 3 from a Huge creature creates a roughly 9×9 block of it. "Takedown" also interacts badly with the updated charging rules. Drag is probably the most useful feature here, minor action shift and drag an adjacent prone enemy 1 square (Requires a prone enemy adjacent). A bit specialised in it's use, but the cold resist, size and Cold keyword on it's attacks might make it of use to a cold themed or Frostcheese using Defender.

Skeletal Steed: (MM2, lvl 3) Another undead mount, and a fairly good one considering it's low level. Speed 8, immune to disease and poison, but lacks the necrotic resistance of most undead. "Death Shriek" is extremely useful – a minor action, recharge 5,6, party friendly will-targetting burst that inflicts a -2 to attack rolls (Save ends). "Mobile Melee Attack" gets a lot more useful with it's rider action "Mount of the Dead" – it's rider can make a melee basic attack as a free action during the move. This is not limited to the same target or timing as the mount's own attack, or you can skip both attacks in order to use a melee attack power. Very handy when you need to reposition during a battle, but still need to get an effect onto an enemy or want to do some damage. Even more so for a Fighter, as it allows you to 'Mark on the Run' as it were.

Skinwing Behemoth: (MM3, lvl 7) Strafe, Flyby Attack and Grasping Talons all stack, meaning you can do a Flyby MBA and have your mount grab the poor thing you just hit. Good way to get someone else off their mount, or just off the ground for a nice long drop. Don't get caught grounded though, Speed 2 (Clumsy) is a huge penalty.

Spirehorn Behemoth: (MM3, lvl 9 Elite) Action Point! Beastmaster is nice, an untyped AC/Ref bonus, and it's Huge. If your DM is willing to let you use an Elite mount, this beasty completely obsoletes the Trihorn Behemoth. It's Trample attack is also fairly powerful, and probably worth using.

Howler Doom Mastiff: (MM3, lvl 13) Guarding and Piercing Spines provide very nice defensive boosts, and Terrifying Howl grants permanent CA as long as you can stay adjacent to the enemy. A very good choice for a melee combatant looking for survivability and CA – Perhaps your Rogue would like a spiky little doggy of his own?

Lolth's Reaver: (MM3, lvl 25) For being so high level, I'm somewhat underwhelmed by this one. (And that's not just because I'm arachnaphobic). Web Saddle is situationally useful, as is the ability to jump 8 squares without provoking OAs, resulting in a blast that prones (save ends), but somehow, it feels like it needs more to truly qualify as an Epic Tier Mount. There are other mounts with better defensive and movement capabilities, either as passive benefits, at-will or recharge powers as opposed to the Encounter power here. Useful to mess up the frontline, or a bit of control, but you can probably do better than this by this level.

Carver: (EPG, lvl 6) Speed 8 and an MBA that causes ongoing 3 damage (Save ends). "Vicious Steed" allows the carver to make an attack when you miss with an OA. Too situational to be really useful, and nothing else about the mount is really worth recommending. Skip this one, unless you're building one of those weird blind auto-miss characters that have been popping up lately.

Clawfoot: (EPG, lvl 3) "Medium Steed" allows a medium character to ride this medium sized critter, intended for small characters. Given that being large or huge is almost always a benefit of a mount, this is of dubious value. "Clawfoot Charge" allows both you and the Clawfoot to make MBAs on a charge. There are better ones for chargers, but it's nice for it's low level.

Dragonhawk: (EPG, lvl 8) Fly 12, Overland 15, an MBA that slides 1 square, and it grants it's rider +2 perception and the ability to roll twice for initiative, using the better result. A decent choice, but be very wary of the speed 3 if grounded.

Fastieth: (EPG, lvl 2) Another "Medium Steed", however this one comes bundled with speed 10, and the ability to shift 4 squares as a move action once per encounter. Good for repositioning, or a caster wanting to stay at maximum range, but of little benefit otherwise.

Glidewing: (EPG, lvl 7) Fly 10, overland 15, with an encounter free action power to gain hover until the end of it's next turn. "Aerial Agility" +1 to all defences for the rider while flying, which is useful. "Diving Smash" again interacts badly with the new charge rules. Reasonable, but a Wyvern is a better deal.

Magebred Destrier: (EPG, lvl 5) A better Warhorse. It's "Trample" can now target any defense (!), the bonus to your charge attack damage increases to +6, and once per encounter, grant it THP equal to your surge value when you spend a surge. A decent compromise between damage boosts and durability.

Valenar Riding Horse: (EPG, lvl 2) A better horse. Encounter power to shift 2 squares as a move action, and a minor action to grant the horse +4 speed until the end of it's next turn. If you're just after a horse, for whatever reason, these will do nicely.
 

DCD: Draconomicon: Chromatic Dragons
DMD: Draconomicon: Metallic Dragons

Adult Pact Dragon: (DCD, lvl 13) (Evil alignment) Fly 10 (Hover), Overland 14 give this mount excellent mobility, made even better with it's "Astral Jaunt", a recharge 6 minor action teleport 10 that takes you along with it. A reach 2 MBA is useful, and because it IS a dragon, it comes with a breath weapon (Recharge 5,6, Blast 5, reflex-targeting, fire damage and ongoing 5 fire save ends), and Bloodied Breath giving a nice out-of-turn attack. Your character took a Backlash tattoo? Well, then this is for you! "Pledged Rider" could save either you or it from dropping to zero, and "Agressive Charger" is another extra attack on a charge, although note that the extra attack differs from it's normal MBA, and lacks the Reach 2. What it DOES benefit from is the "Skirmish" feature, a 2d6 damage bonus to it's melee attacks if you move more than 4 squares away from where you started – easy to do on a charge or if you use "Astral Jaunt". Resist 10 fire, 10 psychic and darkvision is just icing. A very powerful frontliner, although obviously fire resistance may cause problems.

Cobalt Dragon-bred Drakkensteed: (DMD, lvl 26) Fly 10, Overland 15, Resist 15 cold (Granted to it's rider with "Draconic Resistance"), darkvision and 2 Cold damaging MBAs are a good start, with "Flyby Attack" adding mobility. Unlike the vanilla Drakkensteed, it's "Fearsome Roar" is not party friendly (although it's rider and deafened creatures  are immune), but the range does increase to burst 4. Unlike the Drakkensteed, however, this one comes with a "Breath Weapon" in the form of an encounter power blast 6 (!) Reflex-targetting Cold damage attack. "Gift of the Dragon" allows you to use either this "Breath Weapon" or "Fearsome Roar" as a free action when you spend an AP. Probably best used by a Cold-themed character (Frostcheese) and ignore it's party-unfriendly dazing capabilties.

Drakkensteed: (DMD, lvl 16) One of the more controller-y mounts out there, and a decent one at that. Fly 10, overland 15, with "Flyby Attack" making it very mobile, but the best part is "Fearsome Roar", an encouter power burst-3, party-friendly save-ends daze, which recharges and is used as a free action when you spend an action point while riding it. Excellent control potential for adding to your arsenal, particularly if combined with save penalties, fear effect support or decent AP benefits from a paragon path. The Skull Mask works well, imposing a -2 to save vs fear effects.

Grave-born Drakkensteed: (DMD, lvl 21) Another controller-y mount, which, while slightly slower than the vanilla Drakkensteed at Fly 8, overland 14, and inflicting weakened instead of dazed, comes with a few other benefits. It's encounter power is a blast 4, inflicts weakend (save ends) and does a reasonable amount of necrotic damage, and once again is used as a free action when you spend an AP. The same comments from the Drakkensteed apply here. Being undead, it has poison and disease immunity, darkvision, and resist 15 necrotic, and "Grave Resistance" grants you resist 15 necrotic and poison. It's MBA causes ongoing 10 necrotic (save ends), and it has an at-will ranged attack that makes the target take a -2 penalty to attacks made against it or it's rider.

Liondrake: (DMD, lvl 12) This one falls somewhere between a charger and a controller's mount, with "Fierce Steed" granting both it and you a +4 untyped to all defences while charging, and "Battle Leap", a standard action to shift 4 squares and use it's MBAs, both of which are Reach 2, but where it really shines is "Terrifying Roar" a recharge 5,6 party friendly burst 5 (!) thunder damaging attack that inflicts a save ends daze, which is again, a Fear effect. Has the potential to trivialise encounters with lucky rolls on it's recharge. Fly 10 (clumsy) is decent, but do mind the -4 to it's attacks and defences while flying (Which negates the "Fierce Steed" bonus for it, but not you as the rider).

Vulture Drake: (DMD, lvl 9) Another clumsy flier (Speed 8 this time). "Bite" MBA for ongoing 5 necrotic (save ends) and a standard action to charge from flying and make two "Claw" attacks, proning if both hit. "Death Shriek" is interesting, an encounter blast 5 will-targeting attack for thunder damage, ongoing 5 necrotic and a -2 to attacks and defences (Save ends both). Powerful, but it's immediate reaction nature, and uncontrollable trigger (when first damaged) limit it's usefulness. "Death Scent" does not benefit the rider's attacks. Has resist 10 necrotic, which it gives it's rider, along with a +4 untyped bonus to saves vs fear effects. It's drawbacks (difficult to use, commonness of necrotic resistance) outweigh it's useful features to me. Avoid.

War Shark: (DMD, lvl 14) I shouldn't list this, but it's here for completeness's sake. It's a Dire Shark, renamed, go read it's entry above.

Compendium links to follow when it is updated with DS content.

Erdlu: (DSCG, lvl 1) The Athasian analogue for a horse, although it's closer to the Eberronian dinosaurs, with Medium Steed. Escape Sprint could come in handy, if the Run action itself wasn't terribad. But still, it's the campaign's relevant lvl 1 probably-always-available creature of burden, and beggars can't be choosers.

Inix Adult: (DSCC, lvl 9) A grab attack and the ability to knock away enemies in a (sadly rather small) blast could be useful, but overall I would say avoid this one. It's mount action is a free action counterattack, but the trigger is horribly situational – most players will avoid being hit with, or even triggering OAs with their ranged/area powers. Pass.

Kank Soldier: (DSCC, lvl 4 Soldier) The ability to grab, grant ongoing damage and a possible immobilise is nice, if nothing spectacular given that it takes two standard actions to do so. Speed 8 and a couple of resistances make for a reasonably mobile and durable low level mount, so you could do worse.
 
Mekillot: (DSCC, lvl 10 Soldier) Again, nothing too spectacular, as it's only really useful feature is the ability to charge through enemies and prone them, then grant you an MBA at the end of it. 1/encounter only to boot. It is Huge though.

War Crodlu: (DSCC, lvl 5) First off, I have to say I find Crodlu to be immensely fun to say out loud. Crodlu? Crodlu! Crodlu Crodlu Crodlu. Try it. Setting that aside, the War Crodlu is a respectable choice for a low level mount, with a 1/encounter mba tagged onto a charge, but the real gem is Pounce – a standard action to move, then make two attacks, 1 for 2d6+3, the other for 1d8+4, which totals to reasonably high single target damage for an at-will ability.
 

As handy as it might seem to have a mount (perhaps a back-up) in your back pocket, remember that these (Unless specified otherwise) consume a daily item use. Also, they do not conform to the normal Mounted Combat Rules:
 

Bronze Griffon: (Dragon 384) Higher level than, and similar to the normal Griffon, lacking it's Overland flight, but still, fly 10 isn't bad. "Aggressive Charger" only grants 1 attack to the Griffon's two, and it's version of "Blood Frenzy" doesn't grant the extra move action, but has an easier trigger of either you or it being bloodied, and grants a better bonus of +2 to speed, fly speed, and attack rolls.  Probably the most combat capable of all the wondrous figurines.

Ebony Fly: (AV, pg 181) Fly 10, Overland 15, and as usual, mind the ground speed of 4. Comes with "Aerial Agility" for that +1 to your defenses.

Jade Sea Snake: (Dragon 381) Swim 10, and one of the few swimmers with a ground speed (Albeit only 6). Huge, and can carry up to 6 characters (Which has interesting implications in combat – multiple move actions, burst/blast/area attacks against it will likely affect the entire party, and the ability for an opponent to pick whoever in the party he wants to attack in melee if he's adjacent) More meant as a form of transport, as emphasised by it's movement-based, and utterly useful rider power "Deft Slither" – neither you nor it (and thusly any of the 6 of you on it) do not provoke OAs for moving while swimming.

Marble Elephant: (AV, pg 182) A higher level Elephant, however, it loses the Reach 2 MBA of it's living counterpart.

Obsidian Steed: (AV, pg 182) This one falls somewhere between a Riding Horse and a full-on Warhorse, as it has a +5 to charge damage, but no "Trample".

Opal Carp: (Dragon 381) Ignore the "Small Rider" section, the rules errata invalidates this, as Large is no longer a requirement. Swim 8, grants it's rider underwater breathing and a better version of Goblin Tactics with "Opal Glamer" – concealment against all ranged and area attacks, and if you are missed by any attack, you get to shift one square a free action. Possibly one of the best mounts to make a quick getaway on.

Pearl Sea Horse: (AV, pg 183) The mount it summons is a higher level version of the Sea Horse, so see it's entry for details. Comes with the ability to breathe underwater for it's rider.

Tourmaline Turtle: (Dragon 384) Technically more a vehicle than a mount. Speed 1, swim 4 , aura 5 of underwater breathing for allies and protection from the environment while mounted on it. It does have a Reach 2 attack, and it is Gargantuan but really, WHY are you taking this into combat? It's meant for planar travel only, regardless of whatever it's fluff text says about considerable power in battle.
 

Adamanine Horse of Xarn: (DMG2) (Artifact) I do not have a copy of DMG2 to hand currently to be able to fully review all it's capabilities, but the mount itself can be summed up neatly as 'A better warhorse', same "Trample" capabilties, and a bigger damage boost to charging, at +10.

Bridle of Conjuration: (AV, pg 169) A useful little item indeed. Summons a Riding Horse (See above), for 12 hours or until it is killed. A handy way to have a back-up mount available, but it does come with the downside of the mount not attacking for any reason. (Not that a Riding Horse's attacks are anything to write home about!) Depending on setting, it may suit your campaign to have it summon a more thematicly appropriate animal, for example the Giant Lizard, Draft.

Figurine of Tantron: (Dragon 368) (Encounter useable, Artifact) This one breaks the Wondrous Figurine rules, but it is an artifact after all. "Stamp" bears the icon of an MBA, so assumedly it can be used as one (and knocks prone too). "Trampling Charge" functions the same as a vanilla Elephant's. Please note that the stat block in the compendium is for Tantron's Pleased level of concordance. The actual article shows it's actual capabilities – when at satisfied or above, it can stay in it's mount form permanently, has the Warlock's Shadow Walk, and "Multiply Tantron's Overland speed by 5". Given that it does not have an overland speed listed, one must assumedly use it's base speed, resulting in 8×5=40, or 20 miles per hour, making it one of the fastest mounts in the game for long-range travel.

Phantom Steed: (Ritual, PHB pg 310) The mounts this ritual summons are not combat capable, but with a sufficiently high arcana check, are capable of some phenomenal speed, with extra movement forms to boot. Any Ritual Caster with a decent Arcana check should keep this one around in a mounted combat campaign, or just for quick A-to-B travel.
 

Impenetrable Barding: (AV pg 123, lvl 1,11,21) Applied as Magical Barding, which means using this can still grant your mount an AC bonus. The property listed has been errated to a flat Resist 5 per tier to all damage while ridden which is still respectable. I would highly recommend all mount users spend at least the 360gp for the Heroic version, if you have no need of the other mount slot of items.

Mirrored Caparison: (AV pg 124, lvl 2) A +1 item bonus to NADs for 520gp is decent, if slightly out-shone in effect as a passive property by the Martyr's Saddle. Unfortunately doesn't scale with level, +1 is all you get. It's At-Will power allows you to make an area attack that includes the mount as a target not include the mount, which can save it a lot of damage, but it does use up your immediate action. It can be used on friendly area attacks, for those riding Huge mounts that might be in people's way. Dependant on how much you need your immediate.

Horsehoes of Speed: (AV pg 123, lvl 3) +1 speed to all your mount's movement speeds. Useful, and a very cheap item at higher levels where a lot of mounts have multiple modes of movement. A solid choice, although how you get horseshoes on a Dire Riding Shark is beyond me.

Saddle of Strength: (AV pg 124, lvl 3) +50% carrying capacity for your mount. Only for use if you're following the encumberance rules, and even then it's largely a waste of the item slot. Pass.

Ghost Bridle: (AV pg 123, lvl 4) Your mount (not you) gains resist 10 necrotic and a daily power to grant both of you phasing until the end of your next turn. Phasing has it's uses, but really dependant on how much your DM uses the Undead. A Skeletal Horse may be a better option at that point anyway.

Bridle of Rapid Action: (AV pg 123, lvl 5) The power on this thing may as well have been a property, as there is little to no reason NOT to use it every encounter. Effectively roll twice for initiative and take the better result. There are initiative related feats that aren't that good.

Martyr's Saddle: (AV pg 124, lvl 6) +1 item bonus to all your mount's defences. Useful, but again, doesn't scale. At-will interrupt to take 1/2 the dmg your mount would take from an attack is a good, if action-costly way to increase it's survivability, but marred by the fact that 'nothing can prevent or reduce the damage a rider takes in this way', which includes having resistance to damage. Best used only for it's passive bonus, or in a dire situation.

Steadfast Saddle: (AV pg 124, lvl 8) Encounter interrupt to negate one pull, push or slide that affects your mount. Too situational for my liking, and there are other powers out there that can do the same thing better, although this will require the ability to target an ally.

Zephyr Horsehoes: (AV pg 124, lvl 9) Your mount ignores difficult terrain and can cross liquid as if it were solid (Lava etc still causes damage). Useful enough, but there are a few mounts that can do this anyway, and many at later levels that can fly. More valuable if, for example, your campaign is limited to non-flying mounts.

Saddle of the Nightmare: (AV pg 124, lvl 15) As worded, the only way to effectively use a mount capable of teleporting. Whenever your mount teleports, willingly or otherwise, you can go with it. Does not apply in reverse, so if you teleport or are forced to, you'll leave your mount behind. Uninteresting, but neccessary by rules. Ask your DM nicely if you can work around this one.

Saddle of the Shark: (AV pg 124, lvl 15) Your mount gains a swim speed equal to it's land speed and both of you gain the ability to breathe underwater (And you can speak normally underwater to boot). Extremely campaign dependant and there are other ways to gain water-breathing. A naturally waterborne mount may be a better option.

Bridle of Flame: (AV2 pg 110, lvl 16) Whenever you spend a healing surge to regain HP, your mount also regains HP as if it had spent one. Considering that mounts, as creatures, lack a decent numer of healing surges, this item is the only way to heal them that doesn't involve surge-less healing or regeneration. Unfortunate, but true. The daily power on this item grants the mount resist 20 fire, which is useful, and a 2d6 fire dmg aura (Do note that this aura targets creatures and is therefore party unfriendly). As a nice side effect, this item is part of the 'Arms of Unbreakable Honor' set, which includes the divine weapliment 'Bradaman's Weapon'. Using these two (or the Unbroken Lance mentioned further down) grants the 2pc bonus, a bonus to speed while charging equal to the number of items from this set you possess, whether you are on the mount or not. A good choice for a Defender's mount.

Skystrider Horsehoes: (AV pg 124, lvl 18) Remember why you bought your character the Zephyr Boots or Airstriders? Exactly the same deal, the mount gains a flight speed (but not hover) equal to it's land speed, several levels below the aforementioned two items. A good choice, but don't forget that many mounts can already fly at this level.

Beast Rider: (Dragon 384) This is the feat that allows a Ranger to ride their beast companion. Obviously, it requires the Beast Mastery class feature. Because your beast companion is now serving as a mount, you may use mount slot items on it, but not at the same time as a companion slot item. Note that unless your beast companion is larger than you, this feat is required to get around the size restrictions in the rules. Technically, by the nature of 'this says you can, otherwise you can't', you still need it for a Large companion, but technically, the rules which say you can mount/ride any adjacent, willing creature allow you to ride your beast companion. At least, I assume your pet Gryphon would be willing? No? Really? Been hitting the dessert tray a bit too much lately and it doesn't wanna carry the extra weight?

Swift Rider: (Dragon 385, Paragon, Elf, Mounted Combat required) +1 feat to your speed and +2 feat to speed for your mount. Again, stacks with mount speed increasing items, and a nice bonus for yourself to boot. Not a bad choice, but does not stack with Mark of Handling's speed bonus.

Valenar Rider Training: (Dragon 385, Elf required) Proficiency with all simple and military spears, +2/3/4 damage bonus with them and shortbows while riding a mount. Reasonable, thematic, but there are better choices for weapons, as most characters will be going for a superior bow or spear. The best feature of this feat is the ability to use Elven Accuracy to reroll attacks your mount makes, but you're more likely to be using it on yourself. Unless going for a very specific idea using a mounts attacks, probably not worth the feat slot.

Spirited Rider: [Arena Fighting] (Dragon Annual 2009, any martial class, Wis 13, Mounted Combat required) Only of benefit to fighters and warlords. You may use Cleave when charging on a mount, and you may use Commander's Strike to order your mount to attack, and push 1 if it hits with that attack. Using an MBA on a charge is always a bonus, and cleave isn't a bad choice – it'sa good way to pop a minion. With regards to the Warlord part, the first part about allowing your mount to make the attack is not needed. Commander's Strike specifies 'One creature' as it's target, with 'An ally' in it's attack line. Mounts are considered your ally. RAW, it implies that it isn't allowed, but RAI, it's unneccessary. Push 1 on a hit could have some use with a mount that prones as part of it's attacks, but there are better ways to do this, usually using your own attacks. Situational at best for a warlord, and you're usually better suited granted the MBA to one of your melee types in the party.

Holy Steed: (Dragon 388, Paladin, Paragon, Mounted Combat feat) Untyped +2 to all defences and +your cha mod to it's damage rolls for your mount while you're on it? Yes please. A solid choice for a mounted Paladin. I highly recommend this for those defending from horseback.
 

The Griffon granted by this path is one of the few 'mounts' to actually scale with level, and be properly implemented in the CB. Fly speed of 8 and low-light vision are decent, but it's a pity it can't hover, which would make it truely amazing for an archer-beastmaster.

The Paragon path itself has several decent features, the best of which is probably it's AP feature, allowing the griffon to take a standard or a move action after the attack (I'm a little confused as to why it is worded this way – a standard action can be downgraded to a move action regardless, and preventing minor actions seems a little pointless.). The bonus damage if the beast attacks is just icing. A utility power to surgelessly heal your Griffon is very, very useful, it's a pity it's daily. The rituals are also a nice bonus, although perhaps slightly under-par for a lvl 16 feature.

Technically, you still require the Beast Rider feat to ride the griffon, but your DM may and should be willing to overrule this.
 

Wizards and sorcerors have several powers that have benefits for having an active familiar, or enemies being adjacent to it,  or it being in the area of a burst, etc. Remember that your familiar is in active mode while it is serving as your mount. I've rated the familiar powers below on the assumption that you're using your familiar as a mount. They will differ in use if not, obviously.