Cerberus

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