Cerberus is the secondary antagonist of SCOOB! He was the guardian of the Underworld in Greek mythology.
In Greek mythology, Cerberus was the son of the monsters Typhon and Echidna and one of many other monsters sired by the two beasts. He became the guard dog of the Underworld when Hades, the god of the dead, came to rule the Underworld and organized it. In this role, Cerberus' task was to keep the living from entering the Underworld and the dead from escaping. He greeted every new soul that entered, but once behind his spiked tail and his dangerously sharp teeth and claws, they could never leave. Cerberus only visited the world of the living once when Hercules came to capture him for his twelfth and final labor.
When Scooby-Doo and Mystery Inc. sought to stop Dick Dastardly from retrieving Cerberus' skulls and opening the gate to the Underworld to retrieve his dog Muttley, Dastardly found all three and coerced Scooby into opening the gate as he was the last descendant of Peritas, the dog who belonged to Alexander the Great. With the gate opened, Cerberus was returned to life and hungrily attempted to devour Scooby and the gang. Blue Falcon helped put up a fight while Fred, Velma, and Daphne sought a way to return Cerberus to the Underworld.
Eventually, Scooby and Shaggy convinced Dick Dastardly's robots; the Rottens, to help them, and they changed into bowling balls to get Cerberus through the gate. Blue Flacon, Dynomutt, and Dee Dee fought the monstrous hound and distracted him long enough for the Rottens to get under his paws. Cerberus lost his footing and tumbled back through the gate, whereon Scooby and Shaggy sealed it together, but Shaggy was to remain in the Underworld. However, Cerberus returned Shaggy to his friends as he was still living, for only the dead belonged in the Underworld. Eventually, the gang caught Dick Dastardly, and Cerberus never again returned to harm the living again.