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The one time the series has ever remotely touched the subject of what Genesect was originally like, it shat on the Kabutops hypothesis with this fossil.
In all honesty, I wouldn't be surprised if what Genesect once was hasn't really been thought about. Game Freak designed a Bug type cyborg transformer, THEN picked the "modified ancient predator" as its backstory. Being an alien like Deoxys, a product made from scratch like Porygon, or a modified Scizor were all also probably considered too but discarded.