Which original Pokémon Game Boy version was not released outside Japan?

Pokémon Green was the original Pokémon Game Boy version not released outside Japan.

In Japan, Pocket Monsters Red and Green launched the franchise in 1996. Nintendo later released an updated Blue version in Japan, with revised artwork and dialogue. The code and presentation of that Japanese Blue version formed the basis for the international Red and Blue releases.

As a result, players in North America, Europe, and many other regions generally encountered Pokémon Red and Blue rather than the Japanese Red and Green pair. The international Blue version should not be confused with the Japanese Blue release, even though both share the same color name.

Pokémon Yellow followed as an enhanced first-generation title featuring Pikachu. It was released internationally and drew more directly from the television anime, including Pikachu’s role as the player’s starting Pokémon.