Which Pokémon game was the first main-series title to feature a Fairy-type?

Pokémon X and Y were the first main-series Pokémon games to feature the Fairy type.

Released worldwide in October 2013 for Nintendo 3DS, the games introduced Fairy as a new type in Generation VI. It was the first new Pokémon type added since the Dark and Steel types debuted in Pokémon Gold and Silver.

Fairy-type Pokémon and moves were designed partly to balance the battle system, especially by giving Dragon-type Pokémon a major new weakness. Fairy is super effective against Dragon, Dark, and Fighting types, while Fairy itself is weak to Poison and Steel.

The games also introduced Mega Evolution, character customization, and fully polygonal 3D graphics in the main series. The Fairy type was not exclusive to newly introduced Pokémon: several older species, including Clefairy and Jigglypuff, gained Fairy typing in X and Y.