Pokemon Green (Gameboy Classic)

Many of you know that in Japan, Pokémon was 1st released in Red and Green versions while Red and Blue versions were released here in the U.S. What very few people don't realize is that the U.S. Blue version is the same as the Japanese Green version. In Japan, their third release, Pocket Monsters Blue version, was quite unique in comparison to the original Red and Green, and also different from the English versions, as well.

What's so different about the Japanese Blue that sets it apart form the Red and Green versions? Well, the most noticeable change was that all of the Pokémon had been redone to look better. (Consequently, these 'redone' Pokémon were also used in the English Red and Blue versions of the game). Another was that Pokémon could be found in places that they originally weren't in in the Red and Green versions. Dittos were added to the Rock Tunnel and Lickitung could be caught in the wild. Also, the Unknown Dungeon was totally redone, with an all new layout (also used for the English versions of the game).

(taken from http://www.serebii.net/green/)