Gameboy Transfer Pak - Nintendo 64
The Transfer Pak is an accessory that plugs into the controller and allows the Nintendo 64 to transfer data between its own games and Game Boy or Game Boy Color games. The Transfer Pak has a Game Boy Color slot and a part that fits onto the expansion port of the N64 controller. It was included with the game Pokémon Stadium, as the game's main feature is importing Pokémon teams from Game Boy titles.
Pokémon Stadium and Pokémon Stadium 2 are games that rely heavily on the Transfer Pak. Pokémon Stadium also includes a "GB Tower" mode for playing Pokémon Red, Blue, and Yellow directly on the Nintendo 64 via a built-in Game Boy emulator (which includes unlockable "Doduo" and "Dodrio" modes which speed up the game by a factor of 2 and 3, respectively).