Cerberus

Cerberus was a three-headed hellhound (the poet Hesiod claimed he had 50) with snake-like extremities, a snake for a tail and the claws of a lion Cerberus was said to have been the offspring of two monsters, Typhon (a fire-breathing serpent) and Echidna (commonly portrayed as an odd juxtaposition of beautiful woman and deadly serpent).Echidna was in danger of being struck down by Zeus who was in the process of ridding the world of monsters, when she went into labor. Zeus had just defeated her mate, Typhon, the most deadly monster in Greek mythology. But as she lay there at his mercy with her newborn baby monsters around her, Hera (goddess of birth, womanhood & marriage) refused to allow Zeus to kill them. Hera’s fervent entreaty worked and Zeus relented, sparing Echidna and her monstrous brood.
In turn, Echidna offered her oldest son Cerberus as a tribute to Hera. Although repulsed by the creature, Hera knew the importance of such a gift and having Echidna as a potential ally. Hera found the perfect solution in gifting Cerberus to her brother Hades. Hades loved Cerberus as beloved pet. As he grew, Hades trained him to be the fiercest of guard dogs.
In adulthood, Cerberus was the watchdog of the Greek underworld and faithful servant to Hades. He was charged with the job of watching over the gates of the underworld — devouring anyone trying to return to the land of the living and refusing entrance to anything alive.