Theory
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.
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?
