fish block

Very rarely while fishing in any biome, you would get a MASSIVE catch that is surprisingly easy. but you wont get a regular fish, you instead get:

BigFin Flounder

(or whatever other name gets decided)

It cannot be cooked into cooked fish, nor can be put into fishing barrels. you place it down like a block and it acts like a fishing barrel, but with 10 slots for holding fish instead of 6.

It can also be sold to willy, in a similar fashion to fishing barrels. willy does not care for the bigfin flounder itself, so you get to keep it.

Below is a quick sketch of what it would roughly look like