Which Pokémon game generation first introduced Shiny Pokémon?

Generation II first introduced Shiny Pokémon.

The feature debuted in Pokémon Gold and Silver, released in Japan in 1999. A Shiny Pokémon is an unusually colored version of a species, with coloration determined by special game data rather than by a separate species or evolution line.

In the original Generation II games, Shiny status was connected to a Pokémon’s individual values, the hidden statistics used to distinguish Pokémon of the same species. The games also gave players a guaranteed encounter with a Shiny Gyarados at the Lake of Rage, making the rare feature visible to many players.

Shiny Pokémon are not necessarily stronger than ordinary members of the same species. Their rarity and alternate colors are the attraction. The term “Shiny” became standard later, but the underlying alternate-color mechanic began with Gold and Silver, not with the original Red, Green, Blue, or Yellow games.