The "Denri Butterfly" is the name of a supposedly exotic butterfly native to the Clark County area. From the various written descriptions, it appears to actually be a "Cairns Birdwing", a fairly common butterfly native to Australia. They must have migrated here with some more human immigrants, a fairly common occurrence for insects. Like many other non-native species, they appear to have been optimally adapted for the area, and have pushed many of the native butterfly species out of the area. Certainly, the Cairns Birdwing has a beautiful and uncommon coloring compared to North American butterflies, so it makes some sense why it would be considered to be a particularly "magical" specimen.
What is less clear is why it took so long to figure this out. Any decent Entomologist with an interest in butterflies should have recognized it. Instead, Caleb Drab, Professor Virgil Pendrake, and other "scholars" of the fauna of Spruce Street have constantly misidentified the butterfly. Everyone even vaguely involved with studying Spruce Street seems to suffer from either insanity, intellectual dishonesty, or incredibly naivety. Why should you trust their accounts of Creeping Bryophytes and Blarnrats when they can't seem to spend the time to do basic research?
- Randy Jameson, "Spruce Street Debunked!"