What will you do with a 40 IV shinny Magikarp?

Sushi

My 5th Gyarados is a 49% B/C 1950. I'm halfway to my 6th and have a 183cp reserved for that slot. Not gonna rush it though, doubt I'll even pinap

So glad I was not in this position. If it was my first one I would like to say I would not evolve it but knowing me I would have. If it was my eighth one I would let it sit as a trophy. I've evolved all my impressives (69-75%)

I would look at it

Now I have fun gym fodder.

Pawn it for coins

Buy a fish-tank or put it in the pond of your front- or backyard to show of.

1) When you fight gyms, if you are not going to need all six attackers, select your golden karp in sixth spot. This will build up its "battles won" number. Then you can put it in a gym and show off in two ways: shiny Magikarp with high battles won.

2) Eventually evolve it and put it in gyms. Maybe fast TM for Dragon Tail, but don't waste any stardust.

This is what I did with my 3 shinykarp - one was CP 124, the others only 10 and 20 (IVs good to below average). Evolve, max out, put in gyms. They seem to encouage rivals to attack the gyms and come home with coins ;)