Board Thread:Speculation House/@comment-14138255-20141017020106/@comment-9090085-20141021004714

Mercen-X wrote: So, anyway, basically Grimm "evolve" in the sense that they change as they grow, but not in the sense that they "adapt to become stronger" and "weed out weaknesses." They may become more powerful and more durable as they increase in age (and subsequently, size), but it is not a case of "oh, that blade cut me, better grow some bone there." It's also not a generational thing unless RT wants to retcon the "Red" Trailer as being Ruby pit against the last remaining ancestors of the current generation of Beowolves (or something). I say this because the original Beowolves are currently the only "proof" that species-wide changes can occur within the Grimm. Well, the Beowolves in the "Red" Trailer could be "newborns" who had yet to grow the masks and spikes of the "children" fought in-series (a little like how a baby turtle has a soft shell but an adult turtle grows a hard shell).

Or, as pointed out before, the appearance of the Beowolves could have been retconed for the show. I do seem to recall Monty mentioning that he wanted to re-make the trailers.