Blastoise was the mascot on the original Japanese Pokémon Blue box.
In Japan, Pokémon Blue was first distributed as a special mail-order edition for CoroCoro Comic subscribers in October 1996, before receiving a general retail release years later. Its updated artwork and programming became important because the international Red and Blue versions were based largely on the Japanese Blue version.
The original Japanese Red and Green games featured Charizard and Venusaur as their version mascots. International Pokémon Blue instead used Blastoise, creating the familiar Red-and-Blue pairing known by many players worldwide.
This history can be confusing because the Japanese release structure differed from the international one. Japanese Blue was not simply the global Blue release under another language; it was an updated version whose technical foundation influenced the overseas games.