The whole? Kyurem came from a metor, it's an alien to the world of pokemon(Though there's been rumors that all pokemon are foreign to the planet they inhabit). Secondly, Reshiram and Zekrom are the two halfs that make the whole. They're like Vanitas and Ventus, only they were seperated because the original dragon was torn between which hero to follow.
Still, Kyurem represents the absence of Yin and Yang, so in a sense it's a nobody, these form indicate that it may want one of the dragons to feel whole, which is where that sense from. Also, I think it's remotely possible that Kyurem will become either way due to using one of the dragon stones. Like how SHaymin would use the Gracidea Flower to turn into it's sky form. It'd make sense, but if these are anything like how G/S/C were it would make you wonder how Zekrom and Reshiram went back into slumber ... Coarse, there is always the possibility this takes place many, many years later on. I doubt it would be a prequel, because they're called Whte2/Black2.
That also makes me wonder ... Are we going to already get new starters 0.o? No, it's too soon ... Right 0.o? Maybe new trainers at best, but I'm not gonna lie, these new forms look amazing, I 'bout had a heart attack when I saw them (Or at least when I got a close look). In a way it's kinda like the shock that Numora has been giving us for a while.
What do think about all this :/?