The Ultra Beasts are a group of extradimensional Pokémon originating from Ultra Space. They possess power that transcends humans' understanding of the world.
Ultra Beasts infrequently appear from Ultra Wormholes in Alola. These appearances are apparently as unexpected for the Ultra Beasts as for Alolans, which tends to make them combative. They are especially drawn to people who have passed through an Ultra Wormhole, due to the residual wormhole energy on them, apparently expecting to find a wormhole they can use to return home.
All Ultra Beasts share the Ability Beast Boost.
In Pokémon Sun and Moon, when encountered in the wild, the residual energy from traveling through an Ultra Wormhole manifests as an aura that boosts one of their stats at the start of battle, similar to Totem Pokémon. Wicke theorizes that they store this energy in their bodies.
Most Poké Balls have difficulty recognizing Ultra Beasts as Pokémon due to their extraterrestrial origins, impairing their ability to effectively catch them. Only the Master Ball and 
Beast Balls escape this impairment.
Although unconfirmed, Aether Foundation scientists hypothesized that Cosmog is also an Ultra Beast, due to its ability to create Ultra Wormholes. Necrozma's Pokédex entry also states that it is "reminiscent of the Ultra Beasts", but when it was in the wild, the International Police reported that all Ultra Beasts it had detected had been caught, and Beast Balls are not effective at catching it.
There are currently eleven known Ultra Beasts:
 Nihilego |
 Buzzwole |
 Pheromosa |
 Xurkitree |
 Celesteela |
 Kartana |
 Guzzlord |
 Poipole |
 Naganadel |
 Stakataka |
 Blacephalon |