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.