Giant Koi is a fish from the Dragon Shrine. It drops worth 94 coins.
It does not resemble the real koi. Its very red, scaly, and big of course.
If you think the big fish are the rare ones, this is NOT rare. The Giant Koi is usually found 249 meters below sea level. Its common on that point. Also since its scaly, the Giant Koi is famous if its real because its second than the Arowana as scaliest and chinese people just make gold relics of scaly fish.
Copied from Ninja Fishing (Updated) Wiki.