The three-headed dog guarding the gates of the Underworld in Greek mythology is called Cerberus. Described as having three fierce heads and a serpent for a tail, Cerberus serves as a protector, ensuring that the dead cannot escape and the living cannot enter without permission. His role highlights themes of life and death, representing the boundary between the mortal world and the realm of the deceased. Cerberus is often depicted in myths involving heroes like Heracles, who encounters him during the completion of his labors, further enforcing the interplay between heroism and the acceptance of mortality.