For future reference, I am only judging based off of the early sprites since that is where any supposed "mix up" would be most likely to happen. Anything after red, blue, green, and yellow would have already cemented the idea of Venomoth evolving from Venonat and Butterfree from Metapod.
Hm... An interesting thought, but personally I doubt it. The similarity only exists if you describe it vaguely... "Round mouth, two teeth, segmented eyes". Their Sugimori art (I've always assumed they were done first, before the sprites, but I'm not sure) looks quite different...
The similarity is very obvious in the original, sucky art (which is sucky because the artist(s) chose very boring poses and had really bad coloring skills), where their faces were practically identical. The sugimori art is newer and based off the FR LF/ RS sprites (I think).
Butterfree's eyes are "cuter", and the two teeth are smaller, and you'd think that was the other way around. Venomoth, on the other hand, has even bigger fangs than Venonat.
In the red/green sprites, the eyes are just as ugly and bug-like as Venonat's. The teeth are smaller, I'll give you that, but I suppose this is a Caterpillar > Butterfly change. The caterpillars with immense spikes don't keep them after turning into butterflies. Venomoth's fangs aren't very visible in the RBGY sprites, as well.
Besides, (inferring again that the Sugimori art came first) you'd think Butterfree would also have brown hands and feet to match Venonat's, instead of changing to blue.
The Sugimori art did not come first; I believe that the sucky art or the Red/Green sprites came first, and I'm not sure if the art or the sprites were first. If we take the sprites to be the original, notice how the legs and arms on Venonat and Butterfree are the same color. I don't know why they're different colors; presumably, the sprites are based off some sketches by Satoshi Tajiri and we don't know what those looked like.
That is pretty weird, and I'd say that it's somethign that keeps me from putting too much faith in this theory.
On that note, Golem doesn't look much like its evolutionary family either, so just the fact that Venonat and Venomoth don't have much in common doesn't mean much. I mean... Geodude and Golem: humanoid faces upon gray boulders, gaining limbs with evolution... and Golem: suddenly losing that new pair of limbs, growing a dark shell and a dinosaurian face. At least Venonat and Venomoth are both purple. :P
It's true that many evolutions look nothing alike, such as Golem and Dragonite. However, Venonat is notable not because its evolution is dissimilar but rather because there is another Pokemon with notably similar features that would make sense for Venonat to evolve into. Like I said, as a kid (and even now), I noticed that Venonat and Butterfree had extremely similar faces whereas Venomoth didn't even have the same eyes. I'm not strong on the 'Venomoth was meant to evolve from Metapod' theory, but they do have the same eyes and Metapod, Caterpie and Venomoth share the same pointy crest on the head. It could all be coincidence, but I think this theory merits more dicussion. ;)
Unless Butterfree is supposed to look like a cuter Venonat, I don't think they were supposed to be related. However, it is still a possibility since a lot of pokemon don't resemble their evolutions(like the aforementioned Graveler/Golem evo, or Rhydon/Rhyperior). But we must remember something: they are both bugs, so they will have some general features, like the mouthes and the eyes. Paras has a similar mouth to Venomoth, and similar eyes, but he doesn't evolve into Venomoth.
Paras doesn't really have that similar a mouth. I mean, I guess you can kind of see the resemblance, but it's not really strong. The eyes are the same, though.
Venonat is pretty darn close to Butterfree, though. The same eyes, same mouth, same antennae, similar feet, and even really similar color are very suspicious. I think there's a strong enough resemblance between Venonat and Butterfree to warrant some more thinking.
