https://www.gamespot.com/articles/new-free-pokemon-available-now-in-ultra-sun-and-mo/1100-6457245/

In addition to this month's free Legendaries for Ultra Sun and Ultra Moon, players can claim three more rare Pokemon right now for no charge, provided you have a Pokemon Bank account on 3DS. The Pokemon Company is giving away the final evolved forms of the Alola starters--Decidueye, Incineroar, and Primarina--as a free bonus for using Pokemon Bank, and these are a little different from the ones you can get in-game.

Unlike typical Alola starters, each of these Pokemon comes with their Hidden Abilities, which cannot normally be obtained in the games. Decidueye's Hidden Ability is Long Reach, which allows it to use moves without making contact; Incineroar's is Intimidate, which lowers your opponent's Attack stat on entering the battle; and Primarina has Liquid Voice, which turns all sound-based moves into Water-type attacks.