While Niantic only mentioned one holiday Pokemon in the December 12, 2016 update, there are actually two available in Pokémon GO right now. In addition to holiday Pikachu, people can also acquire holiday Raichu. If a player collects enough candies to evolve a holiday Pikachu, the ensuing Raichu will also be wearing the same hat.
Game Informer has a video showing one of Pokémon GO‘s holiday Pikachus evolving into holiday Raichu.
Pokémon GO is available for Apple iOS and Android devices. Holiday Pikachu will be available in the wild until 10am PT/12pm CT/1pm ET on December 29, 2016.