Magikarp is a beloved Pokémon, despite (or because of) it's lack of prowess on the battlefield. Magikarp's most famous move would become more and more confusing with each new generation of Pokémon video games, due to a mistranslation that happened during the Pokémon Red & Blue era.
Magikarp had one of the first signature moves in the series, in the form of Splash. The point of Splash is that it's a joke move that deals no damage, which ties into Magikarp being one of the most useless Pokémon in the series. Splash has remained a useless move throughout the series, but it briefly gained a power boost in Pokémon Sun & Moon, when it could be enhanced into a Z-Move called Z-Splash, which raised the user's Attack stat by three stages.
The Splash name evokes images of a fish splashing around in the water, which fits Magikarp's gimmick well. A problem arose in Pokémon Gold & Silver when non-Water-type Pokémon (like Hoppip) gained access to Splash. The Hoppip line consists of Grass/Flying-type Pokémon, which doesn't mesh with the meaning behind the Splash name. As time went on, more Pokémon gained access to Splash, like Spoink, Buneary, and Mimikyu, to the point where the name no longer made any sense.
The reason for the mix-up is due to a mistranslation during the days of Pokémon Red & Blue. According to Bulbapedia, in Japan, Splash is actually called Hop, with the idea that the Pokémon is just hopping about on the ground. The Splash name was likely chosen because it matched Magikarp's gimmick and it was the only Splash user in the game, so no one anticipated that it would become an issue further down the road. As time went on, the name stuck and lots of non-Water-type Pokémon are splashing around in the Pokémon world.
The English translations of the Pokémon games have generally been top-notch and the Splash mistake was one of not anticipating Game Freak's future plans for the series. If Pokémon Red & Blue were the only games in the series, then there would be no issue. The fact that Pokémon continued (and will likely continue for many years to come) means that Splash will be used by all kinds of Pokémon in the future.
Source: Bulbapedia