The dog that guards the gates of the underworld in Greek mythology is Cerberus. This three-headed canine is tasked with preventing the escape of souls from the underworld, represented as a fearsome and loyal guardian. Cerberus is often depicted as fierce and intimidating, symbolizing the finality of death. He embodies themes of loyalty, as well as the barrier between life and death. The figure of Cerberus appears in various myths, indicating his integral role in the journey of souls within the realm of Hades.