- A horse doubles your speed when in combat, we're not worried about realism this is an abstraction - horses make you faster.
- Give it baselines stats of appropriate level, large or regular - you can decide how substantial the horse is.
- Armour barding for a horse is the same as a shield for a humanoid. An abstraction but simple. +1 AC. By extension its also possible to get magical barding surely?
- Consider giving a warhorse a bonus to AC and Damage (+2 AC and 25% additional damage)
- If you riding a warhorse roll a d6 at the beginning of your turn, if its below the escalation die then it makes an attack against an engaged enemy.
- If you or the horse is staggered then you need to make a check to control the horse, this should be hard difficulty if on a regular horse, average difficulty with a warhorse. Failure results in the horse rearing and trying to bolt etc, perhaps even a Dex/Str check may be required to hang on - falling off could hurt.
- Stunts such as jumps and other tricky manoeuvres may required a Dex/Str background check to pull off successfully. Remember failing forward and offering tough choices on failure.
- You still have to disengage with a horse, the GM makes the call whether AOO are aimed at the horse or the rider.
- Missed attacks - Roll a natural 1-5? ignore any powers triggered by the attack and roll it again against the horse.