From Nookipedia, the Animal Crossing wiki
The Coelacanth (pronounced ˈsiː.lə.kænθ, "seel-uh-canth") (シーラカンス, Shīrakansu) is a rare fish. The coelacanth is worth 15,000 Bells and can be found only in the ocean, when it is raining or snowing. Despite having a description regarding it as nocturnal, in reality, it can be caught both day and night. In Dōbutsu no Mori, the coelacanth is the only fish found in the ocean.
 Donation to the museum
 In Animal Crossing
Blathers will say:
"Hoo, my goodness! Glorious! Seen in this light, of course, it's quite a grotesque beast. And yet, it does have a certain peculiar allure, wot? You may rest assured that we shall treat it with much affection and respect, wot! My word as a gentleowl."
 In Wild World
Blathers will comment on the coelcanth:
"It was once thought that the coelacanth had gone extinct, eh wot? Indeed, right up until a scholar saw them lined up in a market. Hoo hoo! That would clearly suggest that they are edible, don't you agree? Though I must admit, they don't look like they would taste very good..."
 In City Folk
Blathers will say the following when given the coelacanth:
"Many years ago, the theory was that the coelacanth had gone extinct...But as it turns out, it still exists and has been living deep in the ocean this whole time, wot? Imagine! This fish has known the world since the dinosaurs... Perhaps it's the true master of the sea!"
 In New Leaf
When donated, the coelacanth will appear in the large tank in the back of the fish exhibit that holds all the large saltwater fish. The exhibit has this to say about the coelacanth:
"Coelacanths are ancient fish once believed to be extinct but recently discovered around South Africa. Called "living fossils," they have apparently changed very little over the past 400 million years. They can live for over 60 years and reach lengths of six feet but have rather small brains, even for fish. They have more fins and harder scales than the average fish, making them a rather resilient breed. Their flavour is very different from most fish, and they contain a fat that is indigestible by humans. It's for that second reason that eating them really isn't highly recommended."
 Encyclopedia information
 Wild World
 City Folk
 Further information
The Coelacanth is an ancient species of fish that was, for a long time, believed to be extinct, due to the discovery of fossils. The first ever live one was caught in 1938. It can grow up to two meters in length and lives for an average of forty-eight years. Coelacanths are found off the south coast of Africa, living in steep rocky shores, sheltering in caves during the day. It can travel around eight kilometers a night in search for food, passively swimming with the current, making it a "slow-moving" fish that only uses its fins to adjust its position. It preys on fish and squid, but some have been known to eat eels, skates, and even sharks. Its main predator is likely to be large sharks.
More information on this topic is available at Wikipedia.
 Names in other languages