Stinkfish

You will pull one of these out while fishing for something else, since the catch rate is low and it shows up across most of the underground rather than in one spot you can target. Do not throw it back. Seafood Dinner is one of the longer food buffs in the game and this is part of the recipe, so a few sitting in a chest save a fishing trip later.

The other recipe it feeds is a novelty potion rather than anything you would bring to a fight, so there is no reason to build a stack for it.