In Naruto, as far as I know,
- 1-tail was captured by Deidara
- 2-tails was captured by Hidan
- 3-tails was captured by either Tobi or Deidara
- 4-tails was captured by Kisame
- 6-tails was captured by the Six Paths of Pain.
- 7-tails was captured by Kakuzu
So by process of elimination, Tobi, Konan, or Orochimaru were responsible for the capture of Kokuo. I ruled out Zetsu because in the data books he was faster than 4th Raikage and as strong as him. Alongside his mist armour makes it quite hard to picture Zetsu killing him.
Who captured 5-tails?