Cerberus

The hound of Hades, son of Typhon and Echidna, who fawns on those who enter and devours those who try to leave; fifty-headed in Hesiod, three-headed with a dragon's tail in Apollodorus and the Roman poets, nameless in Homer. Heracles dragged him up to Eurystheus and brought him back; the Sibyl drugged him for Aeneas.