Cerberus

In Greek mythology, Cerberus (Greek: Κέρβερος, Kérberos; Latin: Cerberus) is the monstrous, multi-headed dog that guards the entrance to the Underworld, preventing the souls of the dead from leaving.

Cerberus - a watercolour picture by William Blake

Often called the "hound of Hades", Cerberus was one of the numerous offspring of the monsters Typhon and Echidna; he is usually depicted as having three heads, along with a serpent for a tail.