In Greek mythology, the three-headed watchdog who guards the entrance to the lower world, the Hades. It is a child of the giant Typhon and Echidna, a monstrous creature herself, being half woman and half snake.
Originally, the dog was portrayed having fifty or hundred heads but was later pictured with only three heads (and sometimes with the tail of a serpent). Cerberus permitted new spirits to enter the realm of dead, but allowed none of them to leave. Only a few ever managed to sneak past the creature, among which Orpheus, who lulled it to sleep by playing his lyre, and Heracles, who brought it to the land of the living for a while (being the last of his twelve labors).
In Roman mythology, the Trojan prince Aeneas and Psyche were able to pacify it with honey cake. (See also: the norse Garm.)