Maybe Gamefreak has something up their sleeve and that there's going to be a Final form for Kyurem. I'm holding my breath, since it's a pretty farfetched idea (3 forms) however it makes sense to an extent.
It was originally 1 dragon, split into 2 with Kyurem being the left over shell. But when Masuda announced that there was going to be2 more forms instead of the anticipated (and intended 1 if you followed the backstory), it really stumped me for awhile. Even its Dex description doesn't match the legend at all.
I think that Kyurem is not part of the true dragon but a dragon that can combine with Zekrom and Reshiram.