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I have this theory that some of the baby Pokémon are alternate designs for their evolutions. In some cases it's obvious the baby mon was an afterthought, like with Elekid and Magby, but there's some odd cases like Gyopin looking more like a Seaking rather than Goldeen, or Pichu looking a lot like that "daifuku" design that Atsuko Nishida mentioned as the first look for Pikachu, before redesigning it. Maybe some of the designs got reused as baby mons down the line?
Also why the fuck does Venonat sprite looks like Butterfree's sprite.