Forty Pokémon were usable in the Japan-only 1998 Nintendo 64 game Pocket Monsters Stadium.
Pocket Monsters Stadium was developed and published by Nintendo and released in Japan on August 1, 1998. It adapted the battle system of the Game Boy Pokémon games into a 3D Nintendo 64 presentation and supported connectivity with Pokémon Red and Green through the Transfer Pak.
The game was originally intended to include all 151 Pokémon from the first-generation games. Technical limitations forced Nintendo to reduce the playable roster to 40 species. The selected Pokémon reflected tournament use and ensured that different types were represented.
This game is often confused with the internationally released Pokémon Stadium. In Japan, Pocket Monsters Stadium was the first title in the series, while the game called Pokémon Stadium elsewhere was released in Japan in 1999 under the name Pokémon Stadium 2. The later game expanded the playable roster to all 151 original species.
The distinction matters when discussing Pokémon’s early Nintendo 64 history: the 1998 Japanese game was a limited first experiment, while the 1999 follow-up became the better-known worldwide release.