Cerberus

Cerberus, in Greek mythology, the three-headed dog who guarded the entrance of Hell, not so much to keep people from going in as to keep them from getting out when once they got there. His duty was not always successfully performed, since Orpheus charmed him by his music, and Hercules dragged him neck and crop to the upper world. Hesiod represents Cerberus as having many heads, but three is the more orthodox number. Some derive the name from Erebus, or darkness.