Rhydon was the first Pokémon designed by Ken Sugimori.
Sugimori was a founding member of Game Freak and the lead artist for the original Pokémon games. Rhydon’s design dates back to the early development of the project, when Pokémon was still evolving from an earlier concept called Capsule Monsters.
Rhydon is a large, rhinoceros-like creature with a drill-shaped horn, thick armor, and a Ground/Rock typing in the original games. Its early appearance reflects the developers’ initial experiments with designing creatures that could populate a game world and battle one another.
This fact is different from several other “first Pokémon” claims. Bulbasaur is first in the National Pokédex, Mew is number 151, and Arceus is described in Pokémon mythology as a creator deity. Rhydon’s distinction concerns early design history, not Pokédex order or fictional mythology.