I heard from this dude swimming in tiny pool outside the ribbon syndicate that there was something in the tiny pond with the bridge and to use my super rod.
So I used it three times on every tile and all I found were level 30-86 magikarp.
That's exactly it - high-level Magikarp.
It doesn't sound that great but you just need to level it up one and you got yourself a high-level Gyarados!!
in that bit of water,using a super rod,you can catch Magikarp with a level 1 - 100.
the thing that lurks in the resort area is a level 100 Magikarp.