There is, in a dark and deep far-off sea, the breed of fish that is processed to make the filling for Army sandwiches.
No-one has ever seen a specimen of this fish in the live state. Few have seen it dead and whole. Most of us only encounter it after it has been mashed to a pulp and smeared between to slices of Lidl's Loaf to make a sandwich.
The Mystery Fish fills a similar ecological niche to the Donner, a medium sized ungulate that is killed and macerated to provide kebab meat.
Some speculate that it is not a fish at all, but is the processed remains of the dead. Others suggest that cat poo might be used as a more nutritious (and fishy) substitute.