https://www.gamespot.com/articles/next-free-legendary-pokemon-for-ultra-sun-and-moon/1100-6456948/

Pokemon Ultra Sun and Ultra Moon players still have a few more days to claim this month's free Legendary Pokemon, Dialga and Palkia, but The Pokemon Company has already shared more details on the next two free monsters. Beginning March 1, players will be able to add another pair of Legendaries originally from Pokemon Diamond and Pearl to their teams: Regigigas and Heatran.

Unlike Dialga and Palkia, which are being given away at GameStop stores, players will receive a download code for Regigigas or Heatran in March's Pokemon Trainer Club newsletter. You'll need to create a Trainer Club account on the Pokemon website and opt-in for the newsletter by March 1 in order to receive a download code. You can create an account for free here.