Coffin Crab

You can no longer fish up the Coffin Crab: it is marked Limited and Unobtainable. It belonged to the Frightful Village, where it surfaced in Autumn on Rainy days, using Gummy Fish as bait. The wiki says it hides in coffin-like shells and comes out only to pinch, which fits the village's spooky theme.

Its record is sparse: there are no weights listed, only an average value of C$500 and 300 base XP. That price ranks 390th of the 742 documented fish, which is comfortably above the typical catch of the list, so it was a respectable if unremarkable prize.

Candle Carp, Ectoplasm Eel, Frankenshrimp, Frightful Crate, Gravestone Stingray and Mourning Manta Ray are the other Limited catches recorded for the same village, so the Frightful Village was a full themed set. Since you cannot obtain it, the only useful angle is a collector's one: if you have one saved from the event, the value does not justify selling it, because you cannot replace it for any amount of coins. If you do not own one, there is nothing to do but read the entry for what it tells you about how the event worked.