I wouldn't be surprised if a number of you have seen this theory before, but from the obvious similarities, it is inferred that Butterfree (Venomoth) was originally designed to be the evolution to Venonat, with Venomoth (Butterfree) being the final evolution of Caterpie. I've cooked up a couple of diagrams for you guys to look at:
Makes sense, doesn't it?
Makes sense, doesn't it?