Cerberus Pups are hounds directly descended from Cerberus. A few of those brave enough to stand before Cerberus have collected his seed and used it to impregnate lesser hounds. The result, no matter if it is a hell hound, blink dog, or common mongrel is a litter of Cerberus Pups.
These foul hounds are intelligent and very cunning, often not revealing their true form until it is too late - like when there master is asleep or in a bad situation, whereupon they shape change to true form and tear the limbs from them and rip out their throat. A few evil and powerful masters have somewhat tamed those they own to be a "good" dog.