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I'll start with one.
Porygon2 and PorygonZ are just attempts at creating Mega Porygon.
Porygon2 was considered a step forward, but research was stolen by Team Galactic who created the Dubious Disk, resulting in a corrupted Porygon.
The PorygonZ Smash trophy states that "A patch is in the works", leading me to believe that this theory may be canon soon, and MegaPorygonZ could be the first man-made Mega Pokémon as of Z version.