In medieval time HORSES DID KICK. THey where trained to do so. THe rider would poke the horses in the ribs with his heels three times and it would Kick backwards. Another thing, a kick from a horse could chatter someones spine or kill him very easy. A horse will never stand still if compleatly stopped as we see in M&B it would jammer and franticly try to get out if surrownded. Thats why spear are best against horses, because you are not so close.