The original Pokémon Red, Green, and Blue games were released for the Game Boy.
Generation I Pokémon games were designed for Nintendo’s Game Boy handheld system. Pokémon Red and Green first appeared in Japan, followed by Pokémon Blue and later international versions known as Pokémon Red and Blue. Their portable format allowed players to trade Pokémon and battle friends using the Game Boy Link Cable.
The games’ success helped extend the Game Boy’s commercial life and made Pokémon strongly associated with handheld gaming. Pokémon Yellow also appeared on the Game Boy, although it added anime-inspired changes such as Pikachu following the player.
The Game Boy Color is sometimes given as the answer because the games could also run on that later, backward-compatible handheld. However, the original platform classification is Game Boy. Pokémon Ruby and Sapphire, by contrast, were designed for the Game Boy Advance.