Nothing really, only the fact that you have it, and it exists.
Chat with our AI personalities
It's a level 100 Magikarp.
Level20, As it always does in every other Pokemon Game.
Magikarp evolve at level 20.
Your best bet would be to use the "all Pokemon shiny" code for your game then go fishing in the Resort Area for the level 100 Magikarp that can be caught there. In Platinum, that is.
I am lead to believe Magikarp evolves at level twenty.