The tiny, immortal hydra is a freshwater animal that can regenerate an entirely new animal from just the tiniest sliver of its body. Usually, it does this perfectly: One foot, one long skinny body, and one tentacled head. But with a single genetic tweak, researchers can create monstrous hydras that sprout fully functional heads all over their bodies appropriate for an animal named for an ancient Greek monster that had somewhere between six and nine heads.