It is a legendary fish that you can obtain in Glass Lake but I find no usage of this fish as stated in the description:
A very rare, legendary creature of the sea. Can be used in combination with other legendary fish to create the ultimate fishing trophy.
However, this is not part of fishing trophy:
It is is not decoration trophy where you salvage it and hang it on the wall.
It is not even ingredient for t5 food
The hell is this fish used for then?