Ultra Beasts are a certain classification of set Legendary Pokemon that come from an alternate dimension, one called Ultra Space. The Pokemon are very strong, not as strong statistically as other Legendary Pokemon, but still far greater than normal Pokemon. They each also have the ability of Beast Boost, a New unique ability that raises th Ultra Beasts highest Stats every Time is knocks out a Pokemon on the opposing team.
There are a total of seven different Ultra Beasts apart of the Seventh Generation; a player will have the chance to catch five of these species in each of the given games in the Post Game Ultra Beast Storyline. The other two, however, that are exclusive to the other game must be traded for in order to obtain. The Ultra Beasts include Nihilego, Buzzwole (Sun), Pheromosa (Moon), Xurkitree, Celesteela (Moon), Kartana (Sun), and Guzzlord. The Legendary Pokemon Necrozma is said to closely resemble that of an Ultra Beast, but it is actually a Legend.