Worth noting that the mount rules in the DMG are almost entirely errata'd.
Mounts
Size: Your mount must be larger than you.
Adjacent: You must be adjacent to a creature to mount it.
Willing: You can use a creature as a mount only if it is willing.
Saddles: The rules assume that you ride a creature with a saddle. If you lack a saddle, you take a -2 penalty to attack rolls, AC, and Reflex defense while mounted.
Mounted Combat: Anyone can simply ride along with a beast of burden without using the Mounted Combat feat. The Mounted Combat feat allows you to make the most of a mount’s abilities. When you have the Mounted Combat feat and you ride a creature, you gain access to any special mount abilities it confers to its rider. (Not every creature has these abilities.) While you are riding a creature with Mounted Combat, the creature can make any Athletics, Acrobatics, Endurance, or Stealth checks using your base skill check bonus rather than its own if yours is better.
Mount and Rider
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.
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.
Mounts in Combat
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.
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 (Adventurers Only): 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. Most commonly, the mount takes a move action to walk or fly, and the adventurer takes a standard action to attack. 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: The mount takes a -2 penalty to attack rolls unless the rider has the Mounted Combat feat. While not being ridden, a typical mount (such as a riding horse) rarely attacks on its own, unless it has been trained for battle, is defending itself, or feels unusually protective of its rider. Left to its own devices, a typical mount avoids combat.
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.
Squeezing: When the mount squeezes, the rider is also considered to be squeezing.
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, 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.
