Cerberus (Gr., Lat.). Cerberus, the three-headed canine monster, which was supposed to watch at the threshold of Hades, came to the Greeks and Romans from Egypt. It was the monster, half-dog and half-hippopotamus, that guarded the gates of Amenti. The mother of Cerberus was Echidna—a being, half-woman, half-serpent, much honoured in Etruria. Both the Egyptian and the Greek Cerberus are symbols of Kâmaloka and its uncouth monsters, the cast-off shells of mortals.
Source: H.P.Blavatsky - The Theosophical Glossary