The three-headed dog that guards the gates of the Underworld in Greek myths is named Cerberus. Cerberus is typically depicted with three fierce heads and a serpent-like tail, serving as a formidable deterrent to prevent the living from entering the realm of the dead. He ensures that the souls of the departed do not escape back to the living world. Cerberus signifies the boundary between life and death and represents the fear and mystery surrounding the afterlife in Greek mythology.