Rhydon was the first Pokémon species designed for the series.
Rhydon was created by Ken Sugimori during the development of the original Pokémon games. It is a Ground/Rock-type Pokémon and occupies National Pokédex number 112. Its early design status is often discussed because it predates the final ordering and presentation of the original Pokédex.
Rhydon became an important visual milestone for Game Freak's creature-design process. The sprite-like concept helped establish the direction for designing monsters that could be represented within the technical limits of the Game Boy while still having distinct silhouettes and battle identities.
This fact is frequently mixed up with Pikachu, which became the franchise mascot, or Mewtwo, which was designed early and became central to the first movie. “First designed” does not mean “first in Pokédex order,” “first revealed publicly,” or “most important character.” In the finished National Pokédex, Bulbasaur is first and Rhydon appears much later.