well, there wernt many knights back then as the number of horses that could handle such a weight was probably very slim. also, why use armor if you are on a horse? maybe arrow protection but i would have thought most "knights" were the jousting hit and run type. i cant really think that there were a lot of super heros that would ride in in full armor to jump off into a melee fray. in either case if there were super armored dudes on horses riding into battle, its for a very short burst and also that horse is in their prime and probably wont ride into another battle.