I could be remembering wrong, but I don't think Fluffy is ever referred to as "a Cerberus" in the book. Cerberus isn't a species anyway; it's a proper name. The correct answer for Fluffy would be "three-headed dog" (or maybe just "dog").
I support Cornflakesfu. "Cerberus" is a proper name of exactly one dog in classical myth that happened to have three heads. It's not like a chimera or a cyclops.
Also, Cerberus had a snake for a tail and other snake-like features. Fluffy was just a big mutant dog.
you cant be a cerberus just as much as you cant be a fluffy.. or a smaug etc.
So @hpaddict, it is definitely not, maybe you should read up on greek mythology ;) and norse while you are at it cause several things rowling put in the potter books have norse origins. (not nearly as much as tolkien has obviously, but that man nearly wrote his own mythology, many of the creatures we are familiar with today, and used so often in books, movies and games, would otherwise have faded into obscurity)
Tenebrous was mentioned during the lesson about thestrals, hagrid taught that lesson. He was also mentioned when Harry and the gang were going to "rescue" Sirius.
Also, Cerberus had a snake for a tail and other snake-like features. Fluffy was just a big mutant dog.
So @hpaddict, it is definitely not, maybe you should read up on greek mythology ;) and norse while you are at it cause several things rowling put in the potter books have norse origins. (not nearly as much as tolkien has obviously, but that man nearly wrote his own mythology, many of the creatures we are familiar with today, and used so often in books, movies and games, would otherwise have faded into obscurity)
I knew about the thestrals, but couldn't recall Tenebrus ever being mentioned. At all.