From Nookipedia, the Animal Crossing wiki
The Monarch Butterfly (オオカバマダラ, Ōkabamadara), also known as Monarch, is a brown-orange butterfly that can be found in September, October, and November. It is very common. Like most bugs, the creature sells for a considerably lower amount than all fish - in this case, lower than all of them. It is the only butterfly to appear during the autumn and one is of four bugs (itself, the Cricket, the Bell Cricket, and the Migratory Locust) to be limited to autumn.
Donating to the Museum
In Wild World
The Monarch Butterfly can be seen in flying around the upper right corner of the first bug room, occasionally perching on flowers.
In City Folk
After donation, it can be seen flying around on the upper tier of the insect exhibit.
In New Leaf
Upon being donated, the butterfly can be found in the room of the bug exhibit which has the light in the middle, resting on the tree which also is home to a Lantern Fly. The exhibit has this to say about the Monarch Butterfly:
"Monarch butterflies are known for their lengthy southward migrations in the winter. Once spring rolls around, they return to their original homes until the weather turns cold again. In their southern habitat during winter, you may see many thousands of them covering the trees."
Bug Encyclopedia information
The monarch, sometimes known as the wanderer, is a milkweed butterfly with a wingspan ranging from 8.9 to 10.2 cm. Native to North America, the monarch can also be found in New Zealand, Australia, Portuguese archipelagos, and occasionally in Western Europe. Aside from their vivid orange coloration, the monarch is famous for its annual migration to and from North and South America, of which no single individual survives — the entire journey takes three to four generations of monarchs to complete. Due to their milkweed diet, the monarch is toxic, or at the very least distasteful to many would-be predators. The highest concentration of toxins can be found in the wings and abdomen of the monarch. The males have a brighter coloration than the females, and are slightly larger. Males also have two small black spots on their rear-wings [see picture], something that is not shared with females.
Names in other languages