What was the first Pokémon spin-off series to feature randomly generated dungeons?

Pokémon Mystery Dungeon was the first Pokémon spin-off series built around randomly generated dungeons.

Its first games were Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team, released in Japan in 2005. Red Rescue Team was made for Game Boy Advance, while Blue Rescue Team was made for Nintendo DS. The paired games reached other regions in 2006.

The series adapts the Mystery Dungeon role-playing format to Pokémon. Players control a Pokémon rather than a conventional human Trainer, explore changing dungeon floors, collect items, battle enemies, and recruit other Pokémon to their rescue team.

The series is distinct from Pokémon’s main role-playing games because dungeon layouts are generated as players explore them. It is also different from Pokémon Ranger, which focuses on real-time capture mechanics, and Pokémon Snap, which centers on photography. The first Rescue Team games established a long-running Pokémon subseries.