There's sooo many. I would pick a theme or try and divide it into sections....even come up with a series maybe? That'd be neat, a whole series focusing on different species or origins... just an idea. Great concept though! I'd love to see some.
^this guy's got it covered. Since its a book about monsters, stray away from things that look too much like humans or too much like a single animal, as those topics have been covered by other sorts of how to draw books. Personally, I'd like to see a manananggal since I'm Filipino, or something like a manticore.