How did apollodorus describe the monster cerberus?

Apollodorus described Cerberus as a monstrous, three-headed dog that guarded the entrance to the Underworld. Each head had its own unique characteristics, with one often depicted as having a snake for a tail and other serpents sprouting from its body. Cerberus was known for his fierce nature, preventing the dead from escaping and the living from entering without permission. This fearsome creature symbolized the boundary between the living world and the realm of the dead.