What monstrous creature guarded the entrance to the underworld?

Cerberus is the three-headed dog from Greek mythology that famously guarded the entrance to the Underworld, ensuring that the dead could not escape back to the land of the living. His role was crucial in the mythological narrative, symbolizing the boundary between life and death. Each of his heads represented various aspects of his duties: one for vigilance, one for ferocity, and one for mortality. In tales of heroism, such as when Orpheus and Heracles (Hercules) encountered Cerberus, his fearsome presence served to highlight the seriousness of crossing into the realm of the dead, thereby reinforcing his role as a formidable guardian.

The other creatures mentioned, while significant in their own myths, do not fulfill this specific role. The Chimera is a fire-breathing hybrid creature; the Minotaur is known for dwelling in the labyrinth; and the Sphinx is famous for riddling travelers. None of these monsters is associated with guarding the entrance to the Underworld in the way that Cerberus is.