Rhydon is often cited as the first Pokémon ever designed by Game Freak.
The claim refers to early development of the original Pokémon games, not to the order in which creatures appear in the Pokédex. Rhydon became National Pokédex number 112, while Bulbasaur received the first number because it was selected to open the final Pokédex list.
Rhydon was introduced in the first-generation games Pokémon Red, Green, Blue, and Yellow. It is a Ground/Rock Pokémon that evolves from Rhyhorn and later gained a further evolution, Rhyperior, in Generation IV. Its drill-like horn, armored body, and dinosaur-like silhouette helped establish the series' early creature designs.
A common mix-up is confusing the first designed Pokémon with the first Pokédex entry, Bulbasaur, or with the first Pokémon shown in promotional material. The wording is sometimes reported as “first Pokémon created” rather than “first designed,” but Rhydon is the accepted answer to this trivia question.