What three-headed dog guards the underworld?

The story behind the answer

Cerberus is the three-headed dog who guards the gates of Hades, stopping the dead from escaping and the living from entering. He’s the correct answer because ancient sources consistently place Cerberus at the Underworld’s entrance, often depicted with a serpent tail and a mane of snakes. In contrast, Chimera is a lion-goat-serpent monster, Geryon is a three-bodied giant, and Orthrus is a two-headed dog.

Cerberus is the offspring of the monsters Typhon and Echidna and famously appears in Heracles’ twelfth labor, when the hero captures him alive. To remember: “Cerberus sees thrice” and “Cere-bus guards the cemetery,” linking his name to death’s domain. His triple heads symbolize vigilant watch over past, present, and future souls.