Horses and the hero shield
by AshtagonA simple way is to let the horse be surrounded by the PC's "hero shield":
* Any attack that hits a horse that is currently being ridden by the PC can, at the PC's option, have the damage transferred to the PC instead.
* An area attack that would by RAW hit both PC and horse instead only hits one or the other, at the PC's option. (This is the same game logic that protects a piece of flammable equipment when the PC gets hit by a fireball.)
Optionally, GMs could require Handle Animal or Ride skill checks (or Wisdom ability checks) to activate these each time the PC wants to use these features.