From Nookipedia, the Animal Crossing wiki
The Yellow Butterfly (モンキチョウ, Monkichō) is a common butterfly that can be found anywhere near flowers. It is worth 80 Bells in Animal Crossing and 90 Bells in Animal Crossing: Wild World and Animal Crossing: City Folk. They flutter slowly around town from March to October. It is very similar to the Common Butterfly, in every way but color. It was the first revealed bug in Animal Crossing: New Leaf, with a photo of one flying near a tree. It has appeared in every Animal Crossing game so far.
Donating to the Museum
As with all bugs and fish in Animal Crossing series, the Yellow Butterfly can be donated to the Museum by talking to Blathers. During each donation, he will enlighten the player to some information about the donation. Below are quotes from each game, on what he has to say;
In Animal Crossing
"There's an old saying among some of my contemporaries: "Butterflies are beautiful, yet moths are monstrous." Well, if you were to ask me, I'd say they were all the same! By which I mean disgusting! Truly hoo-rrific, wot wot!"
The Yellow Butterfly can be found fluttering around the flowers in the central display in the insect room after donation.
In Wild World
Upon donation, Blathers the curator speaks of the the Yellow Butterfly;
"Butterflies are covered in "scales" that flake off like dust when touched, eh wot? Blech... The very thought makes my eyebrows itch unbearably... Anyhoo, I suppose that's neither here nor there..."
As with other butterflies, the Yellow Butterfly can be found flapping around the flowers in the first room of the insect display.
In City Folk
Once again, Blathers will speak of the Yellow Butterfly in a rather horrified way:
"We call them all yellow butterflies, but many subspecies of this insect exist, wot wot? Indeed, this fact makes them rather interesting to many researchers... What's more interesting to me is why anyone in their proper mind would take interest in these things."
In City Folk, the Yellow Butterfly appears flying around the flowers on the lower terrace of the insect room.
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 Yellow Butterfly:
"The yellow butterfly is similar to the common butterfly, with the two easily mistaken for one another. Males are more often a yellowish color, and females are more often white. The larvae feed on plants of the Fabaceae family, such as milk vetch or white clover."
Bug Encyclopedia information
Pieridae, yellow-white butterflies, are a large family of butterflies that are widespread throughout the world. So common in fact, that many sub-species have been given dozens of names throughout the world. Yellow-white butterflies are, as the name suggests, yellow or white in coloring, and usually have black markings on the wings. Males are generally colored differently to the females of the same species.
The butterflies shown in Animal Crossing are females, as males lack the yellow spots within the black bands on the edges of the wings. Colias erate is locally known as the Eastern Pale Clouded Yellow and is widespread over eastern Europe and Asia.
Names in other languages