I don't know if I made a thread suggesting this before, but I've brought it up numerous times on the forums. Stakes were used throughout history by archers as crude protection from horsemen. It wouldn't necessarily physically stop a horse (i.e. the horse's leg might get fucked up and eventually kill the horse, but it's body might still fly into you as it fell), but it would certainly damage it.
This should be part of the siege equipment for the game, and like people said, even if it doesn't actually hurt a horse, it would be nice if it would physically stop them from riding through it. Lots of times the stakes were very crude and just fashioned near a battle from tree branches that were sharpened into points. But it did the trick.