The first Pokémon Stadium offers only about forty playable Pokémon and a handful of modes, most of which require a Game Boy cartridge. Here is the list of Pokémon and the game’s modes.

Pokémon Stadium, Japan, 1998.

The modes

All modes except Battle require a Red, Green or Blue cartridge saved in a Pokémon Center and connected through the Transfer Pak.

ModeContent
BattleFree battle or tournament, against the computer or other players.
StorageMove Pokémon and items between the party, the PC boxes and the game’s boxes.
ListView your Pokémon and their stats.
PokédexThe Pokédex in 3D, with all 151 Pokémon.
RegistrationRegister a team.
TeamExamine the current team.
GBPlay the Game Boy game on your TV, the ancestor of the Game Boy Tower.

The playable Pokémon

Only these Pokémon can battle. For the most part, they are the most used Pokémon in the official tournaments of the time, rounded out to balance the types.

Venusaur.
Charizard.

Venusaur, Charizard, Blastoise, the three legendary birds and Dragonite are not available in the L1-30 tournament. According to Poképédia, Mewtwo and Mew are also among the available Pokémon, which brings the total to 42.