From Nookipedia, the Animal Crossing wiki
The Napoleonfish (ナポレオンフィッシュ, Naporeon Fisshu) is a big, blue, rare fish with a large humped head. One of a group of fish which all sell for 10,000 Bells, this fish appears during late summer in the ocean and island ocean. Like most other oceanic fish, the Napoleonfish appears in the back tank, when placed in the museum. Its first appearance is in Animal Crossing: City Folk and also appears in Animal Crossing: New Leaf.
Donating to the Museum
Donating in City Folk
Upon donating the Napoleonfish to the museum, Blathers will comment:
"Hoo, my good gracious! What size! What majesty! What impact this fish has! When fully grown, the body of this fish can reach a length of over 80 inches, eh wot? The beady eyes and pouty mouth make it appear a bit dull witted...which I find rather charming!"
The Napoleonfish can be found lazing around near the bottom of the ocean fish tank at the back of the fish exhibit.
Donating in New Leaf
After donating a Napoleonfish to Blathers, it will appear in the section for fish and deep sea creatures, specifically in the large tank in the northeastern room of the fish and deep sea creatures area.
The exhibit has this to say about the Napoleonfish:
"Called Napoleonfish, these fish also have the more accurate name of humphead wrasse. The true name comes from the distinctive humps on their heads. But "Napoleonfish" is more popular. When small, all Napoleonfish are females, but some become males as they age and their humps grow. Their bodies also turn a bluer shade as they mature and become males, making identification simple. Their bulky bodies, which can get to about six feet long, still glide easily through warm tropical waters. Finding these fish is a real treat for divers, as they're stunning to see-- and it's a great story to tell."
The Napoleonfish is actually called the humphead wrasse, and males are larger than females. The fish, unfortunately, is endangered. It is one of the stranger fish that change gender as they grow older.
Names in other languages