Here is what I meant (it's a bit different than the purple one by Duuude007, I think the white on the bottom is just smoke). I've put an arrow at the right horn that appears to be in the fog, however it might be a result of the OPs photoshopping - it's much harder to see in the original. There is writing on top of where the left horn would be to the bottom left of leviathan (I drew that dotted horn a bit big now that I look at it). The middle horns/crown would all be behind the other primals.

original images to compare


But if that right horn is from the OP's photoshopping and not in the original, then I would say it's not Atomos. It's only a "what if" based off an unclear photoshopped image, so I'm just acknowledging it's possible. I do believe Atomos will play a bigger part in ARR though, even if it's not in this poster.