Rhydon is identified in Game Freak accounts as the first Pokémon species designed for the original games.
Rhydon was created during the development of the first Pokémon titles and became an early example of the monster designs that would populate the games. Its design is recognizable for a rhinoceros-like body, armor plating, and a drill-shaped horn.
The claim concerns design history, not Pokédex order or the first creature a player encounters. Rhydon’s National Pokédex number is 112, so it appears well after the first starter Pokémon in the numbered list. It also evolves from Rhyhorn, a related species that was included in the original generation.
Pikachu is often guessed because it became the franchise mascot, while Charizard is frequently associated with the original box art. Those roles came later in Pokémon’s public history and do not make either species the earliest documented design.