Well, from what I believe and understand, Kyurem is a sort of shell of what was Reshiram and Zekrom, before they split. In it's Dex entry it says:
"It generates a powerful, freezing energy inside itself, but its body became frozen when the energy leaked out."
Ah, I just noticed something as I wrote that XD Reshiram and Kyurem both symbolize energy, both natural (fire) and industrial (electric). Kyurem symbolizes an empty shell and a lack of energy, explaining the Ice type.
And I'm trying not to ride the hype train on the R/S stuff, despite rubies and sapphires being all over the game titles