Cerberus was the hound of Hades’, the god of the Underworld. He was guarding the entrance to the underworld to prevent the dead from exiting. Cerberus was usually depicted as huge and monstrous dog with three heads and snakes on its tail and mane.
According to the Greek poet Aeschylus, Cerberus was born out of the Giants Typhoon and Echidna. Cerberus used to be a frightful opponent of Greek hero Heracles, because, on his labours, Heracles had killed Cerberus' three siblings, the Lion of Nemea, The Lernaean Hydra and the dog Orthus.