If you trade in 5 magikarp you get a gyarados
He's in rank S on bright beach, as the boss. I want to get him too, to get dialga to get mewtwo. But it's really hard. The game won't let him fall down so I can pick him up. His body just keeps on turning into a coin. Maybe in the next rank he won't be a boss and just be a regular toy in the game. Unfortunately, I haven't made it to the next level so I wouldn't know.
In ex he's in the wild
I bot him at the red roof store