Ultra Beasts are a certain classification of Legendary Pokemon from The Alola Region, ones that are said to come from an alternate dimension called Ultra Space. These certain Pokemon are very strong, not as strong statistically as other Legendary Pokemon, but still far greater than normal Pokemon. All Ultra Beasts also have the ability known as Beast Boost, a unique ability that raises said Ultra Beast's highest stat every time is knocks out a Pokemon on the opposing team.
There are a total of eleven different Ultra Beasts (not including Cosmog) apart of the Seventh Generation; however, due to Version Exclusives, a player will only be able to find eight Ultra Beasts via Wormholes inside of Ultra Space. The other three Ultra Beasts, unfortunately will have to be traded for in order to obtain them.
The classification of Ultra Beast includes the likes of Nihilego (UB Symbiont), Buzzwole (UB Absorption - Exclusive To Pokemon Ultra Sun), Pheromosa (UB Beauty - Exclusive To Pokemon Ultra Moon), Xurkitree (UB Lighting), Celesteela (UB Blaster - Exclusive To Pokemon Ultra Moon), Kartana (UB Blade - Exclusive To Pokemon Ultra Sun), Guzzlord (UB Gluttony), Poipole (UB Adhesive), Naganadel (UB Stinger - Poipole's Evolution), Stakataka (UB Assembly - Exclusive To Pokemon Ultra Moon), and Blacephalon (UB Burst - Exclusive To Pokemon Ultra Sun).