Cerberus was an enormous three headed dog who guards the gates to the Underworld, keeping dead souls from escaping.
As one of the children of Typhon and Echidna, Cerberus is the only one who works for the Olympians directly, being fiercely loyal to Hades. Cerberus has three heads and one body. One of the labors of Heracles was to bring Cerberus from the Underworld to the Earth above. Hades agreed to let Hercules take Cerberus back to the world of the living, if he could remove Cerberus from the Underworld, with only his brute strength and without killing him. In that myth, when he took him out of the living world (after a long and fierce battle), Eurystheus immediately told him (by messenger) that to complete the labor, he had to return Cerberus, immediately.