Theory
with how rushed pokemon's firs gen was because the studio was running out of money, i wouldn't be surprised if this was a mistake they knew about, wanted to fix, but just had to roll with because it wasn't the end of the world
That does make more sense.
Ummm, actually… Based on the internal IDs, the Caterpie line was added all at once way after Venonat and later Venomoth. If there had been a readjustment or error, you’d expect these not to be in sequence (like with Wartortle losing its evolution and getting the first-design-phase Blastoise in its place). They may have decided to scrap Venonat, but then needed a prevo for Venomoth and kept it in despite the similarities to Butterfree, but having an evolutionary link between Metapod and Venomoth seems highly unlikely.
Interesting, that's probably why the theory remained a theory
I prefer the dratini->gyrados / magikarp->dragonite pipeline theory, myself, but I can gestate with this caterpie branch. The venonat->butterfree coloring and eyes do seem too coincidental. Maybe the artists that worked on each one had their favorite colors, so pokemon they did all ended up schemed similarly?
