From Nookipedia, the Animal Crossing wiki
The sea butterfly (クリオネ, Kurione) is a species of mollusk that live in the sea. They can be caught in the during winter and are the only winter fish that cast such a small shadow in the sea and so, can be identified easily.
 Donating to the museum
 In Wild World
"The sea butterfly is in the same family as snails, as you may or may not know. You might think it's strange to have a snail without a shell, eh wot? Indeed, well, slugs are also snails without shells! I must admit, being lumped in with slugs probably angers the sea butterfly.
The sea butterfly is placed in the large ocean tank in the aquatic display in the back room.
 In City Folk
It occupies a large glass tube in the lower-right corner of the fish exhibition, with other small oceanic fish.
 Encyclopedia information
 Wild World
 City Folk
 Further information
Sea butterflies can be found in the cold waters of the Arctic Ocean, Antarctic Ocean and in the North Atlantic; mirrored, in-game, by their appearance during the winter months. They can grow up to 5 centimeters in length.