Who is the dog guarding the underworld?

The dog guarding the underworld in Greek mythology is Cerberus. Often depicted as a three-headed dog, Cerberus is tasked with preventing the souls of the dead from escaping and keeping the living from entering the realm of the dead. He is a fierce protector of Hades, the god of the underworld, and is known for his loyalty and ferocity. Cerberus is commonly associated with themes of death and the afterlife in various myths.