When a player is riding a mount in combat, mostly to make things easier, they (as the player not necessarily as the character) have total control of the mount.
Just because the horses are doing what they're told doesn't mean that they aren't wild eyed and freaking out about it. They can say that the mount is panicked, and play that out however they wish, having it hold back, or saying that it's moving forward but only unwillingly.
Mechanically however, mounts can be intimidated just like anybody else once they're bloodied. They've only got a will save of 10 which is only an Intimidate check of 25 (assuming that none of the bad guys speaks horse).