In Greek mythology, the monstrous watchdog of the underworld. He was usually said to have three heads, though Hesiod says he had 50. Heads of snakes grew from his back, and he had a serpent's tail. He devoured anyone who tried to escape Hades's kingdom, and he refused entrance to living humans, though Orpheus gained passage by charming him with music. One of the labours of Heracles was to bring Cerberus up to the land of the living; after succeeding, he returned the creature to Hades.