Dark was one of the new Pokémon types introduced in Pokémon Gold and Silver to help balance Psychic types.
The second generation added both Dark and Steel types. They were designed partly because Psychic Pokémon had been especially dominant in the original games. Dark-type moves are super effective against Psychic Pokémon, giving players a new strategic way to challenge them.
Dark Pokémon were also associated with nighttime, trickery, and villainous teams in the games’ presentation. Examples introduced in Generation II include Umbreon, Houndoom, Murkrow, and Tyranitar’s Dark typing.
The Fairy type did not exist until Generation VI, beginning with Pokémon X and Y in 2013. Ghost was already present in Generation I, while Dragon was also part of the original type system. That makes Dark a useful anniversary-era fact because it marks one of the earliest expansions to Pokémon’s battle mechanics.