Great purple emperor

The Purple Butterfly is the rarest butterfly and fastest in and, selling for 2,000 Bells. It is easily noticeable.

Donating to the museum
As with all fish and bugs in the, the Purple Butterfly can be donated to the Museum by talking to Blathers, who will also tell the player a little bit about the donation. Regarding the purple butterly:

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!"