NOTE! Possible spoilers.
Check below. The top pictures show nothing extra, but the bottom ones show "ropes' from tail to back and their arms, antennas, & tails are glowing. Are these extra forms?? Notice these physical changes are permanent on the sprites. In B/W, Zekrom's & Reshiram's tails would glow, but that's it. Completely COSMETIC.
I posted about these possible forms when info was released on BulbaNews, but the post was promptly taken down. I don't know whether the info was wrong or it was never supposed to go up. They called them Overdrive Statuses.