While Cerberus, one of Echidna's children, is said to be fierce enough that the gods fear him, and flesh-eating, he is a watchdog in the land of the already dead.
One of Labors of Hercules was to fetch Cerberus. Unlike the countryside devastating monsters that Hercules destroyed, Cerberus was harming no one, so Hercules had no reason to kill him. Instead, Cerberus was returned to his guard post.
