Cerberus

Cerberus is the multi-headed hound that guards the gates of the Underworld in Greek mythology, preventing the dead from leaving and the living from entering. Most commonly depicted with three heads, though some accounts describe him with fifty or even a hundred, Cerberus was the offspring of the monstrous Echidna and Typhon, making him sibling to the Hydra, the Chimera, and the Sphinx. His three heads were said to represent the past, present, and future, and serpents writhed among his mane and tail. According to myth, only a handful of figures ever managed to bypass Cerberus: Orpheus charmed him to sleep with his lyre, Heracles dragged him to the surface as his twelfth labor, and the Sibyl of Cumae fed him drugged honey-cakes to allow Aeneas passage. In some traditions, the mere sight of Cerberus was enough to drive mortals mad with terror. The creature has endured as a symbol of guardianship and the impassable boundary between the world of the living and the realm of the dead.