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

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.

That does make more sense.