Rhydon was the first Pokémon created by Game Freak, according to the franchise’s development history.
Rhydon was designed by Ken Sugimori, the artist who helped establish the visual identity of the original Pokémon games. Early development sketches for the project that became Pokémon include Rhydon-like imagery, making the creature an important part of Pokémon’s design history.
This fact is different from National Pokédex order. Bulbasaur is number 001, but that does not mean it was the first creature designed. Likewise, Mew is number 151 and is sometimes called the ancestor of Pokémon within the fictional story, which is a separate lore concept.
Rhydon eventually appeared as Pokémon number 112 in the National Pokédex. It is a Ground- and Rock-type Pokémon that evolves from Rhyhorn and later gained the evolution Rhyperior.