Japan’s 1998 Nintendo 64 game Pocket Monsters Stadium allowed 40 Pokémon to be used.
Pocket Monsters Stadium was developed and published by Nintendo and released in Japan in 1998. It adapted the battle system from the Game Boy Pokémon games and let players use the Nintendo 64’s 3D graphics for Pokémon battles.
Although the Game Boy games contained 151 species, the original Japanese Stadium release supported only a limited pool of 40. Players could connect compatible Game Boy games through the Transfer Pak, allowing them to view, organize, and battle with Pokémon from those games.
The limitation is an important distinction from the later Western Pokémon Stadium release. That version followed in 2000 and allowed all 151 original species, while the Japanese title is effectively an earlier, more restricted entry rather than the same game with identical content.