Here's another clue for the sprite-swap theory.
In japanese Venomoth is called "Morphon" モルフォン - a name relate to the Morpho genus.
https://en.wikipedia.org/wiki/Morpho_(genus)How do you recognize the Morpho genus? Well, from the iridescence of their wings and their black outlines
I WONDER which pokemon fits this description, it's such a mystery