Which two games first introduced shiny Pokémon?

Pokémon Gold and Silver first introduced shiny Pokémon.

A shiny Pokémon is an unusually colored version of an ordinary species. The term refers to its rarity and alternate coloration, not to a separate species or a change in its basic statistics. In battle, the game signals the encounter with a distinctive sparkle animation.

The feature debuted in the second generation, making the red Gyarados encountered at the Lake of Rage one of the best-known early examples. That Pokémon is guaranteed by the story, while most shiny encounters are exceptionally rare random events.

Shiny Pokémon were not present as a formal feature in Pokémon Red and Blue. Later games retained the concept and expanded ways to find or increase the odds of encountering alternate-color Pokémon, but Gold and Silver established the tradition.