https://www.gamespot.com/articles/lots-of-free-legendary-pokemon-will-be-available-f/1100-6456193/

Pokemon fans were treated to a regular stream of free items and Pokemon for Pokemon Sun and Moon in the months following their release, and it looks like the series' latest installments, Ultra Sun and Ultra Moon, will likewise enjoy a lot of support over the next few months. The Pokemon Company is dubbing 2018 the "Year of Legendary Pokemon," with a ton of Legendary-themed Pokemon products and giveaways on the way this year.

Throughout 2018, The Pokemon Company will give away free Legendary Pokemon for the series' seventh generation titles. Beginning in February, players can receive a new Legendary in Pokemon Sun, Moon, Ultra Sun, or Ultra Moon each month depending on which version you play. The Pokemon you can get and the games they debuted in are: