Cerberus is the three-headed dog that guards the gates of the Underworld in Greek mythology.
Known as the hound of Hades, Cerberus prevents the dead from escaping the realm of the Underworld. Ancient descriptions vary: Hesiod called him fifty-headed, while later Greek and Roman writers usually described him as three-headed, often with a serpent’s tail and snakes growing from his body. The familiar three-headed version became the standard image.
Cerberus is especially associated with Heracles’ twelve labors. His final labor, in many versions, was to capture Cerberus and bring him temporarily into the world above without using weapons. Heracles subdued the monster and later returned him to Hades. In another famous story, the musician Orpheus charms Cerberus with music.
Cerberus is often confused with Orthrus, another monstrous dog in Greek mythology. Orthrus was generally two-headed and guarded the cattle of Geryon; Cerberus guarded the Underworld. Fenrir belongs to Norse mythology, not Greek mythology.