In Greek mythology, Cerberus, often called the “hound of Hades”, is a monstrous multi-headed dog, who guards the gates of the underworld, preventing the dead from leaving. He was the offspring of the monsters Echidna and Typhon, and is usually described as having three heads, a serpent for a tail, with snakes protruding from various parts of his body. Cerberus is primarily known for his capture by Heracles, one of Heracles' twelve labours. (source)