Who invented the Pokemon mudkip?

Pokémon are created by a team of designers and artists at Game Freak, the company that develops the Pokémon video games.

While you can't pinpoint one individual as the inventor of Mudkip specifically, you can attribute its creation to the overall efforts of the Game Freak team working on the third generation of Pokémon games (Ruby, Sapphire, and Emerald), which is where Mudkip was introduced.