What three-headed dog guards the entrance to the underworld?

Cerberus is the three-headed dog that guards the entrance to the underworld.

In Greek mythology, Cerberus is often called the hound of Hades. His main duty was to prevent the dead from escaping the underworld. Later descriptions usually give him three dog heads, a serpent for a tail, and additional snakes around his body. He was said to be the offspring of Echidna and Typhon, making him the monstrous sibling of the Hydra, Orthrus, and Chimera.

Cerberus is best known from the final labor of Heracles. The hero was required to bring the creature out of the underworld without using weapons. Different ancient authors tell the episode differently: Heracles may overpower Cerberus by strength, receive permission from Hades, or use special protection before returning the dog.

The number three is traditional but not universal. Hesiod’s Theogony describes Cerberus as having fifty heads, while Pindar gives him one hundred. Most later writers settled on three, which became the familiar version in art, literature, and modern popular culture.