Just a random thought, but has anyone noticed how Kyurem's tail is similar to that of Reshiram? It looks like something should plug in there, like it's missing its icy "flame/generator" equivalent. He and Zekrom also seem the share the lines that span their bodies. The overall body of Kyurem seems to match that of Reshiram and Zekrom, as far as proportions go. Large head, thin neck, smaller arms with huge legs and big toes/claws. But Kyurem looks hunched over...as if he's weakened, or perhaps, burdened by the weight of the ice.
My speculation is that Kyurem was indeed the original dragon. He would have been rather magnificent; bulky, yet elegant. A crown/crest (similar to the ice helmet he has now) would be on his head, and he would have six limbs. Wings, arms, and legs (going off of his current image, which has broken wings, arms, and legs). He would have a large mist of cold air/ice billowing out from his tail, as well as a cold air/ice "mane" or sorts flowing from between the "horns" of his crest. He would be mostly grey, but with black and white accents here and there. If anyone's interested, I can try to make a sprite of this and post it here later ^^;
There is also the energy theory; electricity and heat are forms of energy. Once those energies are released, they would leave nothing. You see, cold is not "negative energy." There's no such thing as cold; there's only an extreme lack of heat. Perhaps when the two energies left the original dragon, not only did it leave him weakened, but it also left him without heat, or "frozen."
Let's compare the energy of the two dragons to "souls" of sorts. These souls are located within the being, from the inside of his body. He would have an immense amount of power flowing throughout his body when he was whole. When the two souls left him, his body "became frozen when the energy leaked out," so to speak. Now there is nothing. No soul (explains the lack of pupils...). No energy. No heat. Absolute zero.
Perhaps when the great divide happened, he was thrown into the atmosphere by the sheer force of the energy leaving him. Imagine a nuclear bomb or splitting atoms. He returned to the earth with such speed and force that the impact created a giant crater in the ground. Now poor Kyurem, the husk of a once great and powerful dragon, wanders mindlessly. He stays near the Chasm, for he knows no other home. He only leaves at night to hunt, making no distinction between human and pokemon prey. Food is food for this soulless beast.
Meanwhile, his two halves hold hands with some random human offspring and bring death and destruction to the world over silly ideals and truths.
Talk about injustice :-/
TL;DR?
Reshiram + Zekrom = Kyurem
Kyurem - Reshiram - Zekrom = Lack of heat + Zombie Kyurem
(Lack of heat = really really cold; therefore, substitute)
Really really cold + Zombie Kyurem = the Frozen Zombie Kyurem we know now
Perhaps when the Light Stone and Dark Stone are brought together, they will merge into the God Stone. When exposed to the energy of the God Stone, Kyurem will regain his former glory as the two energies return to him and return life to him. However, since the three have been separated for so long, they have actually become their own physical beings, and the energy is merely borrowed. The three need each other in order to maintain balance. If one dies, the trio becomes disproportionate and the other two would either explode, implode, or peacefully dissipate. Kyurem is merely the absence AND presence of both energies. You know, the whole yin-yang-wuji thing. The last part is mainly just to take care of the fact that Reshiram and Zekrom would have to disappear in order to properly "return" to Kyurem, and I'm sure some people would be rather irritated that their precious dragons vanished.
I'm sure that the restored Kyurem wouldn't be quite his original self. He'd have armor and accents made of ice, his crown would be close to the helmet he has now, and his wings would probably be "finished" or "fixed," seeing as they end rather abruptly now. He'd look pretty awesome with the way I see it in my head I'll try to at least post drawings if I can't get sprites done any time soon ^^;
(sorry if this is too long or done wrong in some form; this is my first post on this site ^^; I joined just to post this )
