"to represent the hound that stood on guard at the gate of Hades"
Relief of Cerberus (Berlin Zoo)
Public DomainRelief of Cerberus (Berlin Zoo) - Credit: Sebastian Wallroth, Wikimedia Commons
In Greek mythology, Hades is the underworld.  It is also the name of the god of the underworld.

Hades is guarded by Cerberus, a multi-headed dog.