A monster that was killed by Hercules and his friend Ioalus. No matter how many of its heads were cut off, it could grow two to a cutted ones place. Cut one head, and two will take its place! Hydra was defeated so, that Hercules cut off heads, and Ioalus burned the wounds shut with a hot iron. Then there was only one head left, which was immortal. It did it very little good, though, because Ioalus and Hercules managed to kill Hydra by throwing piles of rocks on top of it.