It's not super deep, but the theory that the Gen 1 Pokemon evo lines were reworked for Butterfree/Venomoth to better conform with children's understanding of metamorphosis in bug species. Based on visual design, some fans speculate that the original evo lines were:
Venonat > Metapod > Butterfree
Caterpie > Venomoth
But then changed to Cat > Met > Butt and Ven/Ven
Idk i read up on it that sugimori did intend it as it ended up but there's enough doubt and prima facie convincing stuff to make it fun, enduring lore
I buy it