It's only in Black and White 2 so you can fuse Zekrom/Reshiram with Kyurem to make Black/White -Kyurem.
You can fuse it when you use the Gene Wedge on it, and on Zekrom or Reshiram, then in the end, you get either Black-Kyurem or White-Kyurem!
...which are activated by using the Gene Wedge on it and either Zekrom or Reshiram, resulting in a fusion that removes Zekrom or Reshiram from the party until the fusion is undone. It will then become Black Kyurem (with Zekrom) or White Kyurem (with Reshiram). -Bulbepedia
Hope This Helped :D