Cerberus

Cerberus was the watchdog of the Underworld, a monstrous hound from the same brood as the Hydra and Chimaera. He had three heads and, according to some accounts, a serpentine tail and a row of snakes growing along his back. He guarded the gates to the realm of Hades where his main function was to stop the dead from leaving, devouring them if they tried, although he was also charged with preventing the living from entering. Orpheus got past Cerberus by enchanting him with music and Heracles had to use brute force to carry him into the world of the living as one of his Labours.