In Greek stories, Cerberus is the huge dog with three heads who guards the way into Hades — the place where the spirits of the dead were said to live. His job was to keep the living out and the dead in. You meet him in myths, in school, and in games and films that borrow the old story. Though actually, English speakers often treat him as a hell-dog, mixing the Greek underworld with later ideas of hell. Writers also stretch the name: a fierce guard at a door can be “a Cerberus,” meaning someone who lets almost nobody through. That is a joke from the myth, not a second animal.

地獄三頭犬、刻耳柏洛斯