I'll agree that the looks of Venonat and Butterfree are VERY very similar, but there's one other thing that struck me as odd; do Bees start off as caterpillars, too? (butterflies do start as caterpillars though, just to cover bases *winkwink* )
Answer to the question, which I slapped myself with, while I was typing up a reply to another thread, (go figure XD ) would be that you can't really go, based on the real world, because if we look at Girafarig; real Giraffes don't have Psychic powers, and Penguins aren't made of Steel.
But really, maybe Venomoth would have been better as the counter-bug for Butterfree, instead of Beedrill. Very likely either having Venonat completely redesigned, or switched to Butterfree's line, but in the end, we really can't do anything about it. Generally, "Where's the "Poison Moth's wings?" can't really be answered, because its a fictional world, that really only has clarification over the Pokemon species, rather than their looks and capabilities.
But who would have thought.. an oddity of the Pokemon world, in the first Generation. I'm surprised people didn't pick up on it sooner. XD It really is an interesting, and scary (in a fascinating sense) thing to come across. The Venomoth/Butterfree thing, that is.