The Giant Koi is a creature in Mini Ninjas.
The Giant Koi starts near the Temple in Flooded Valley. It stays there and wanders around in the surrounding water. If Hiro goes near the group of smaller Koi in the lake, he can see the fin of the Giant Koi and if he fishes for it he can see its eyes and body come out above the water slightly.
A Tengu wants to see his uncle, but the Giant Koi keeps pestering him and stops him from crossing the lake. After Hiro visits the uncle at the Temple, he gets the sturdy fishing rod from the Tengu. Hiro must try to catch the koi in the same way he catches smaller fish. The koi will drag him all over the Flooded Village until it takes him through the gates and returns to starting position. The koi cannot be caught.
If Hiro attempts to use the basic fishing rod to catch the Koi, the fishing rod will break and cannot be used again.