From Nookipedia, the Animal Crossing wiki
The Emperor Butterfly (モルフォチョウ, Morufochō), also called Emperor, is an uncommon butterfly that is found around flowers in the summer months, late in the day. It is the only butterfly that can be found in the night. It is sold for 2,500 Bells.
Donating to the museum
As with all fish and bugs in the Animal Crossing series, the Emperor Butterfly can be donated to the Museum by talking to Blathers, the curator. When donating, Blathers will say the following upon receiving the Emperor Butterfly:
In Wild World
If you wish to keep an emperor butterfly as a specimen, remove the body. If you don't, oils from the expired body will leak out, changing the wing color. Blech, the very thought of those post-mortem oils...
After donation, the Emperor can be found fluttering around the flowers in the first room of the insect exhibit.
In City Folk
"The emperor butterfly's beautiful blue markings make it a popular gift in certain parts of the world. Hoo, yes, wonderful gift... Such gift givers should have their heads examined, I should think..."
It can be found in the bottom terrace, flying near or resting on the flowers with any other butterflies.
In New Leaf
Upon being donated, the butterfly can be found in the first room of the bug exhibit resting on flowers with the other butterflies. The exhibit has this to say about the Emperor Butterfly:
"Emperor butterflies are easily recognized because of their lovely, iridescent blue wings. Pigmentation isn't actually what causes the color, but rather the reflections on their wing scales. Since their wings shimmer when moving, noticing them is much easier when they are midflight."
Bug Encyclopedia information
Morpho menelaus, more commonly known as the Blue Morpho, is an exotic butterfly that can be found in Central and South America. It feeds primarily on rotten fruit, but in the larval stage, frequently eats other larvae of the same species. Oddly, it is not referred to as an Emperor Butterfly in the real world, but the family to which it belongs, Nymphalidae, is sometimes known as the Emperors.
Morpho stems from the Greek word for form, and therefore means 'changed'. This may be a reflection of the species' iridescence, which has the quality of changing colors in the light. Menelaus was a legendary king of Sparta who married Helen of Troy.
Names in other languages