Native to Greece, three-headed dogs are very rare creatures. Interestingly, each head serves a different purpose. They are extremely teritoral yet loyal making them the best guard dogs in the animal world. However, their great weakness is the inability to resist falling asleep to the sound of music.
Unlike, it's ancestor this beast has substantially less heads and has a much nicer demeanor. Instead of guarding the gates of the underworld, it has devoted it's life to protect mankind from the fires of their own making. The only other relative of cerberus I've heard of was a three headed dog owned by a half giant in england. As an employee of a boarding school, it does seem odd that he had 20 foot tall three headed dog on campus.