The mythical creature that guards the gates of the Underworld in Greek mythology is the Cerberus, a three-headed dog. Cerberus serves as the protector of Hades, ensuring that no souls can escape and preventing the living from intruding. This creature embodies the themes of death and the afterlife, emphasizing the barriers between the mortal realm and what lies beyond. Cerberus is often depicted in mythological tales that explore the nature of mortality and the unknown.