What was the three headed dog that guarded hell?

The three-headed dog that guarded the gates of the Underworld in Greek mythology is named Cerberus. He is often depicted as a fearsome creature with three heads, a serpent for a tail, and multiple dog-like features. Cerberus's primary role was to prevent the dead from escaping and to ensure that the living did not enter the realm of the dead without permission. He is famously associated with the hero Heracles (Hercules), who was tasked with capturing him as one of his twelve labors.