Actualy,Skitty(;)),you have to find two,Reshiram and Kyurem.Kyurem is in the giant Chasm in the cave where you battled White Kyurem(did you do that yet?),and after you catch/defeat it, there will be DNA Splicers where it was standing, which allows you to fuse with Reshiram. Reshiram is in Dragonspiral Tower after you bring the Light Stone there.
You must defeat or catch Reshiram to find Kyurem in the cave. After you've gone through the hassle of catching legendaries,fuse Reshiram and Kyurem to...get...the almighty..........WHITE KYUREM!!!
Hope I helped!