Magikarp’s natural predator is everything. Or nearly everything. Humans don’t eat it because there’s not much meat to it, and it takes a particularly strong set of claws, teeth, or attacks (or a particularly hard rock) to get past magikarp’s scales. But it can be done, and practically everything that considers itself a predator might eat magikarp if they’re looking for an easy meal.
It’s actually one of the reasons why gyarados are so notoriously angry and vicious when they evolve.