https://www.gamespot.com/articles/pokemon-sun-and-moon-players-can-soon-get-ashs-pik/1100-6453182/

Pokemon Sun and Moon players can soon add another free Pokemon to their collections. To commemorate the series' upcoming 20th anniversary film, Nintendo will be distributing six Pikachu, each one wearing the different caps Ash donned throughout the anime series.

Beginning September 19, Nintendo will release a special code via Pokemon.com that allows players to receive one of the aforementioned Pikachu. Each one will only be available for a week and, along with wearing a different hat, comes with a slightly different moveset that reflects the attacks it used at that point in the anime. However, players will only be able to redeem the code for one of the Pikachu, so you'll have to choose carefully. You can find the distribution schedule (and the corresponding games that season was based on) below.