Butterfree definitely wasn’t meant to be the final evolution of Caterpie.
Here's Caterpie. Aw! It's a cute little caterpillar.
The design is pretty standard, but note the greens, yellows and beady black eyes.
Nintendo
Caterpie evolves into Metapod, which is based off of a chrysalis. Even though it looks radically different, several of Caterpie's design elements are still in place.
It's still green with beady eyes.
Nintendo
And then here's Butterfree. Wait, what? What the fuck happened?!
All the design elements are gone! Where did those teeth come from? There are no pupils! It's not green or white or yellow! What's happening?
Nintendo
