Kyurem

Kyurem

Once you've caught (or knocked out) Reshiram or Zekrom (depending on your version) after beating the game, you can return to the back of the Giant Chasm where you fought against Black or White Kyurem and Ghetsis.

Giant Chasm
Kyurem Kyurem  Lv. 70  

Capturing or defeating Kyurem will cause it to leave behind the DNA Splicers — an item that can be used to fuse Kyurem with either Reshiram or Zekrom.