Coelacanth Animal Crossing: New Horizons is one of the oldest fish you can catch in the game.
The coelacanth has also been known as a "living fossil" in multiple previous iterations of the game. It is an extremely rare saltwater fish that only spawns under specific conditions. When caught, it's appearance is a large black and white speckled fish with blue tinted fins. There is one fin on the top of its back and two on its belly in addition to the two on its sides.
Here's how you can add a coelacanth to your personal aquarium.
Coelacanth Animal Crossing: New Horizons: How to Catch
Coelacanth only appear in the ocean when it's raining or snowing. Otherwise, they can appear all year and all day—a change from previous titles such as New Leaf and Wild World. It is classified as a five star "Very Rare" fish. An individual can be sold to the Nooklings for 15,000 bells while CJ will buy it for 22,500 bells.
Its shadow size in New Horizons is "Huge" which makes it extremely distinct when searching for it. In the wild, adult coelacanths can grow up to 67 in. According to Blathers in Wild World, scholars had once thought this species of fish had gone extinct.
Be warned that a coelacanth can only spawn once per gaming session, even if it isn't caught. Players should be careful and take extra precautions if they believe they may have spotted one off the shore.