The horses and donkeys all start clumping together in one corner when placed in a pen. They all go in the same direction and eventually all clump together. When cows are placed in a pen they all go in the same direction but when met with resistance for a while they eventually disperse. This doesn't happen with the horses and donkeys. Keeps happening even if you lure them away with hay. Doesn't happen with any other mobs, just horses and donkeys.