The three-headed dog in Greek mythology, known as Cerberus, is significant as the guardian of the underworld. Cerberus prevents the dead from escaping and the living from entering the realm of the deceased. Each head symbolizes different aspects of the underworld: past, present, and future. Cerberus embodies the complexity of life and death, reflecting fears regarding mortality and the afterlife. Its representation also highlights themes of guardianship and the boundary between worlds.