Monster with nine heads that ravaged Argus. If one head was cut off, two new grew out, thus making the monster impossible to kill. One of Heracles labours was to kill the Hydra, and he managed it by having his companion Iolaus burn each stump after he had cut the head off. When all the heads were off, Heracles buried the Hydra under a pile of stones, and dipped his arrows into the monsters blood, making them poisonous.