It's nature's disguise. Predators will be tricked into thinking that's the horse's eye, and will attack the wrong end of the critter. That's about when the horse will swing around and whack the attacker with his shillelagh (or whatever that thing is that the horse has concealed there under its belly).
I am laughing my ass off you guys! I noticed you befriended your own self too Trix...now thanks to JP, I must ponder that...
Damn this is a funny photo!!!!!!!!!!!!!!!!!!!!!!!!