Butterfree did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Butterfree is a dual Bug/Flying type Pokémon species that greatly resembles a butterfly. If a Butterfree flaps its wings hard enough, the wings will begin releasing toxic dust. Butterfree is the final evolution of Caterpie and evolves from Metapod. It was introduced in Generation I, and debuted in the Game BoyPokémon originals Pokémon Red & Blue.
Butterfree are pale blue, have 4 legs, and are incredibly large (3 feet tall/1.1 meters), unlike real-life butterflies. Their eyes are large and red, and their wings are composed of scales.