Cerberus

In Greek mythology, Cerberus, often referred to as the hound of Hades, is a multi-headed dog that guards the gates of the underworld to prevent the dead from leaving. He was the offspring of the monsters Echidna and Typhon, and was usually described as having three heads, a serpent for a tail, and snakes protruding from his body. Cerberus is primarily known for his capture by Heracles, the last of Heracles' twelve labours.