A hellhound is a mythological hound that embodies a guardian or a servant of hell, the devil, or the underworld. Hellhounds occur in mythologies around the world, with the best-known examples being Cerberus from Greek mythology, Garmr from Norse mythology, the black dogs of English folklore, and the fairy hounds of Celtic mythology. Physical characteristics vary, but they are commonly black, anomalously overgrown, supernaturally strong, and often have red eyes or are accompanied by flames.

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.