At the Paramount Pictures Comic-Con panel Abrams had this to say: 

His quick speech suggests that it's only one monster, and that it's something completely original. This doesn't rule out Rampage entirely, as the possibility of multiple monsters may still be one of the movies many secrets, and the design of the creatures from Rampage could still be "insane" and "intense" but- holy shit, I'm doing it right now aren't I? 

Anyway, to dive into the sea of theories and viral marketing, try any of these sites. A warning however; it can get addicting, especially since the trailer is so goddamn intriguing, and the creators are being incredibly tight-lipped about the whole thing. Personally, with the movie still several months away, I don't think I can torture myself any longer. It'd be like waiting until next year to see another episode of Lost ...