Cerberus is the original Hellhound of Greek myth; it is a massive, three-headed 'Demon-Dog'. Cerberus is currently in the ownership of The Grim Reaper. How Cerberus ever came into Grim's ownership was never addressed during the series. It was evident from Cerberus' debut in "Mandy Bites Dog" that Grim couldn't control him, claiming that Cerberus, "didn't just eat [Grim's] homework, [Cerberus] also ate [Grim's Dad]" (though it was probably just an exaggeration on the script writer's part). While all three of Cerberus' heads have refused to obey Grim, (prefering to chew on Grim's bones,) they have since been disciplined by Mandy and her rolled-up newspaper.
Cerberus has two shepherd (or more likely, doberman) heads (one brown and one red) and one poodle head, (it is believed that the poodle is a girl and the shepherds are boys because the poodle didn’t completely fall for Little Delicious like the other two did). Cerberus’s three heads all have different personalities. Cerberus can breathe and urinate flames. A special whistle can be used to call Cerberus from where ever he is, and when this whistle is blown, Cerberus appeared in the form of a comet with a tail.
Cerberus married Little Delicious, a small Chihuahua that was owned by Mindy at the time. Together, Cerberus and Little Deliciouse had a puppy that was found by Billy, a three-headed demon Chihuahua that he named "Little Chiba".
Cerberus makes another appearance later in the series, playing 'fetch' with Grim and Billy, with Grim sending Cerberus' toy ball flying by hitting it with his scythe. Cerberus has seemed to have become more tame since his encounter with Mandy.
Cerberus also makes an appearance during the opening credits of Billy and Mandy's Big Boogey Adventure, when he appears to have three heads that are all the same, accompanied by the lyrics; "Cerberus, my lapdog, as loyal as can be".