Rhydon is widely credited as the first Pokémon species designed for the franchise.
Rhydon was created by Ken Sugimori during the early development of the original Pokémon games. It appeared in early development material and was used as a placeholder or test creature while Game Freak worked on the game's systems. The design's dinosaur-like body, horn, and rocky armor made it suitable for testing battle graphics and Pokémon illustrations.
Rhydon is a Ground/Rock-type Pokémon and evolves from Rhyhorn. In the original games, it is associated with the Safari Zone and can also be encountered in other locations depending on the version. Its early design history predates the public release of Pokémon Red and Green.
Pikachu is often assumed to be the first Pokémon because it became the franchise mascot, but Pikachu was designed later. Bulbasaur was one of the first three starter choices, and Mew was created late in development as a hidden extra. “First designed” refers to development history, not the first Pokémon listed in the Pokédex.