https://www.gamespot.com/articles/pokemon-go-christmas-event-now-live-adds-more-gen-/1100-6455777/

Pokemon Go's second Christmas event has now begun. Like last year's holiday event, it introduces new Pokemon to the popular mobile game, makes it easier to get your hands on Pokemon Eggs, and brings back a festive Pikachu for a limited time.

For the duration of the event, Ice-type Pokemon will appear more frequently in Pokemon Go, and players will have another chance to encounter a Santa hat Pikachu. Like last year, it can evolve into a Santa hat Raichu, and this time players have a chance to hatch a Santa hat Pichu, as well. Niantic is also adding an additional 20 Ice- and Water-type Pokemon originally from Ruby and Sapphire to the game, including Milotic, Sharpedo, Wailord, Walrein, Relicanth, and more.