In Greek mythology, the three-headed dog that guards the gates of the Underworld is named Cerberus. This fearsome creature has the duty of preventing the dead from escaping and the living from entering without permission. Cerberus is often depicted as a fierce protector, embodying the threshold between life and death. His role underscores themes of death, afterlife, and the inevitability of fate, reinforcing the mysteries surrounding the Underworld's realm in Greek tales.