The 2014 Kentucky Derby just wrapped up, and the horses were looking pretty darn ripped. What's amazing is that this enormous, powerhouse species evolved from a timid, dog-sized animal named Eohippus. This equine ancestor roamed the lush forests of the Eocene period for many millions of years before gradually Hulking out with the rest of the Neogene megafauna. Horses were big animals by the time humans learned to domesticate them, but now with selective breeding, they have become true giants. Eohippus was a cutie, no doubt. But their descendants could kick 'em clear to Mars.