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?
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.
