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Maybe it has something to do with the show. Maybe they wanted Ash to have a caterpillar, that evolves into a chrysalis, that evolves into a butterfly. Rather than either a gnat thing that evolves into a butterfly, or a caterpillar that turns into a moth. Either way, you end up with a mon that kids won't get as attached to, and won't love the 'Bye Bye Butterfree' episode as much.
I dunno. I'm probably overthinking it.
