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Are we sure that Mitsuboshi was actually a beta Ledian at this point?
Maybe both Ledyba and Mitsuboshi were early attempts at a Volbeat/Illumise combo, where they weren't technically the same line but were just very similar pokemon? They do have different learnsets for level up, aside from being a ladybug, this fucking thing doesn't actually look very much like beta Ledyba or Ledian (Ledyba is green and Mitsuboshi is red, though Ledyba and Ledian were eventually made red) and it would definitely explain Ledian's shit stats if it was meant to be a pseudo-standalone pokemon. Plus, the only other Pokemon that is definitely supposed to evolve into another Pokemon in the demo that doesn't is beta Hoppip.