broken remains of the original dragon which Zekrom and Reshiram were part of. That's why it's Pokedex entry says that its power of ice overwhelmed itself. It couldn't control its power with only one of the original 3 parts of the dragon left behind, and this left it broken and weak. Here is Drayano's evidence:
Now, can you see the evidence for Kyurem being related to Reshiram and Zekrom? There are two particularly big things I can point out. The first is the colours; Reshiram is White, Zekrom is Black. What colour is Kyurem on the whole? Grey. What colour is between Black and White? Grey. The colour certainly matches, giving it a possibility of being the legendary the third version of B/W focuses on. When you consider the last two third version mascots have been the third legendary of the trio, it seems very likely Kyurem is the third of the trio. There's also one very obvious link - the music that plays when you fight Kyurem. Most legendaries have a standard theme, but there are three distinct legendaries that have their own theme, namely Reshiram, Zekrom and Kyurem! The three of them share the same music (with some very slight differences in the background), further solidifying the theory.
Of course, compared to Reshiram and Zekrom, Kyurem hardly looks as imposing. There's a reason for this sighted ingame; legends tell that Kyurem's incredible powers of ice overwhelmed it and caused it to freeze itself, severely weakening it. You might also notice that the left 'wing' is broken; it's shorter than the other one, and quite stumpy. In fact, Kyurem barely looks alive at all. Could Kyurem be the broken remains of the dragon that Reshiram and Zekrom were once part of? There's also the question found in some simple mathematics; adding Kyurem's base stats together gives you a total of 660, whereas all powerful legendaries - including Reshiram and Zekrom - are of the 680 base stat total (with Groudon and Kyogre being notable exceptions at 670.) Kyurem is definitely weaker than the other two, and the reasons for it being so will probably be explored in the third version. It might even get a new forme, fixing those base stats as well as giving it the so far unobtainable Ice Burn and Freeze Shock moves. Of course, it's all conjecture, but the evidence is quite strong.
So you see what I mean now?