What is the name of the three-headed dog that guards the underworld?

Cerberus is the name of the three-headed dog that guards the entrance to the Underworld in Greek mythology. This creature is often depicted as having three heads, a long tail, and sometimes multiple serpent-like appendages. Cerberus's primary role is to prevent the dead from leaving the Underworld and to block the living from entering without permission. He is the offspring of the monster Echidna and the giant Typhon, making him a formidable guardian.

The other options represent different figures in mythology. Hades is the god of the Underworld itself, and while he is associated with Cerberus, he is not the creature that guards the gates. Detritus does not have ties to major mythological figures or concepts. A Griffin is a mythical creature, typically depicted with the body of a lion and the head and wings of an eagle, but it is not connected to the Underworld or its guardians. Therefore, Cerberus is the correct answer as it directly identifies the legendary creature tasked with guarding the Underworld.