Cerberos / Greek mythology

Cerberos is the name of the three-headed dog that guards the gate of the world of the dead in the underworld ruled by Hades in Greek mythology. He is born with the mating of the fire-breathing giant Typhon, both of whom are monsters in Greek mythology, and the half-woman, half-snake Echidna.

It is thought that the three heads of the creature, which is the brother of Orthus, the two-headed demon of hell, symbolize the past, present and future. Although the 8th-century BC Greek poet Hesiode claimed that the beast had 50 heads—except Horace, who later claimed it had 100—it is usually depicted as having three heads. Cerberos does not let anyone out while allowing all spirits in.