Cerberus (Greek: Kérberos) is the name given to the entity which, in Greek and Roman mythology, is a multi-headed dog which guards the gates of Hades, to prevent those who have crossed the river Styx from ever escaping. <a href="http://en.wikipedia.org/wiki/Cerberus" target="_blank">http://en.wikipedia.org/wiki/Cerberus</a>