Rajah Brooke's birdwing

The Raja Brooke Butterfly (アカエリアゲハ, Akaeriageha), sometimes shortened as the Raja B. Butterfly, is an uncommon insect that is introduced in City Folk. It is the only butterfly to be attracted to water, rather than plants. It is easier to catch between morning and noon, and harder to catch later on during the day.

In City Folk
Upon donating to the museum, Blathers the curator will say;

"...I must say, I'm quite impressed with your ability to catch such an impressive butterfly... The Raja Brooke butterfly, I must tell you, is one of the larger tiger butterflies. Naturally, its antennae and proboscis also rank as some of the largest around. Be not proud, vile beast! Your cute red collar and fancy name do not fool Blathers!"

It can be found in the bottom-left enclosure in the insect exhibit, flying around the pond.

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 Raja Brooke Butterfly:

"Raja Brooke butterflies have red heads and a red strip around their necks, giving the look of a collar. They are known to drink hot-spring water and often hang around water flowing up from the ground. While males are vibrantly green, females are much more commonly seen with brown wings."

Further Information
The butterfly is actually a type of birdwing. It has distinctive black and electric-green markings, which are more prominent in the males, while the females have white markings. Both have a red head and another strip of red afterward, which is called a collar. It was named by Alfred R. Wallace in 1855, after James of Sarawak, the Rajah of Sarawark. It is also called the Rajah Brooke butterfly, sometimes mistaking the Animal Crossing spelling as a mistake - though both are acceptable. They can be found in the rainforests of Borneo and the Malaysian Peninsula, and is featured as the national butterfly of Malaysia. It is registered as being under protection.