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Any pokemon with crap base stats has the potential to become a Magikarp type, although there's a chance they'd merely become some souped-up version of their base form
When Mega Evolving, there's a slim chance any pokemon can become a Magikarp - type
Yes, even Mega Garchomp and Charizard, but they won't get the same moves, and if one Mega pokemon gets the Magikarp type, then so will the opponent, to prevent unfair matches in comp play
In the games, however, only the player can wield it