If you're referring to Animal Crossing, it is a very rare fish that can be very hard to catch, but it's worth a lot if you sell it.
Lobe-finned fish were prehistoric fish from the order of Crossopterygii. These type of fish were discovered in fossils with no known live specimen at the time of the discovery. However in 1938 a fish species called Coelacanth, which was thought to have been extinct, was discovered off the Southern coast of Africa. More recently, in 1997 and again in 1998, a separate breed of Coelacanth, named Indonesian Coelcanth (Latimeria Menadoensis), were discovered living in the oceans off of Indonesia. The Indonesian Coelcanth (Latimeria Menadoensis) is officially listed as "threatened", not extinct. All species of lobe-finned fish are believed to be exctinct except for the Coelacanth and Sulawesi Coelacanth.
i just saw it on nat geo, but they actually have never found it. they've seen fossils of it from billions of years ago and it's thought to be the oldest species that we've found to date, i forgot what its called i think it's a type of worm and it lives off bacteria deep in trenches and near sulfur deposits.