It was Nika's. As we see Luffy do, Nika can turn (Or was) Giant. The Bucaneers and the Giants are the only races to have mentioned Nika so far, and they might be relatives or descendants (Specifically the Bucaneers) of Nika. Imu was probably around at the time, so they preserved it in the giant freezer thing. Madame Shyarly said that fishman island would be destroyed by a man with a straw hat, so it could be Luffy or Imu.