Which Pokémon spin-off game was the first to feature Pokémon in a fully 3D battle arena?

Pokémon Stadium was the first Pokémon spin-off game to feature Pokémon battling in a fully 3D arena.

Released for the Nintendo 64 in Japan in 1998, Pokémon Stadium presented battles with polygonal models, animated attacks, announcer commentary, and multiple tournament cups. It was designed to connect with the Game Boy Pokémon games through the Transfer Pak.

The Japanese version originally focused on battles involving the first-generation Pokémon. The international release, titled Pokémon Stadium, included all 151 Generation I species and allowed players to use Pokémon from Red, Blue, and Yellow through the Transfer Pak.

Pokémon Stadium is sometimes confused with Pokémon Colosseum or Pokémon Battle Revolution because those later games also centered on three-dimensional battles. Pokémon Stadium came first and established the console battle format. Pokémon Snap, released later in 1999, used 3D Pokémon environments but was a photography game rather than a battle title.