Ultra Beast

The Ultra Beasts are major antagonists in Pokémon Sun and Moon and Pokémon Ultra Sun and Moon. They are a race of hostile creatures that come from a dimension known as the Ultra Space. It is unclear what their true motives are, but they draw the attention of Lusamine, due to their great power.

Prior Events
Prior to the main events of Sun and Moon, the Ultra Beasts had appeared long ago in Alola. According to the legends, it is said that the Guardian Deities battled these creatures to protect Alola.

Since then, the Aether Foundation devoted their research on these beings. During this time, they worked on a project called the "Beast Killer", where they were developed a new Pokémon to battle the Ultra Beasts called Type: Full. At least three Type: Fulls were made, but when all three went berserk, they were put into stasis, and renamed as Type: Null, deeming the project as a failure. The Aether Foundation also developed special Pokéballs called Beast Balls, which were specifically made to capture the Ultra Beasts.

Meanwhile, the International Police were also investigating the Ultra Beasts. They assigned a team called the UB Task Force to study the creatures and protect them or destroy them. A small group that consisted the members Looker, Nanu and an unnamed person were investigating on a Guzzlord. The Guzzlord was destroyed, but it killed the unnamed member with it.

Pokémon Sun and Moon
The first Ultra Beast, Nihilego, is encountered at Aether Paradise during the player's first meeting with Lusamine. It eventually disappears from the scene, leaving the group in shock.

Later on, Lusamine reveals her true colors and her plan of harnessing the powers of the Ultra Beasts. She uses Cosmog to create Ultra Wormholes across the entire region, spawning out different kinds of Ultra Beasts. After Lusamine is defeated in battle with the player, she escapes to the Ultra Space with Nihilego and Guzma.

The player and Lillie eventually confront Lusamine again in the Ultra Space where she has become completely influenced by the neurtoxins of Nihilego. Lusamine fuses with it, then engages the player in a battle once more. She is defeated, but is not out cold, until she is blasted away by Lunala, the final evolution of Cosmog.

After the player becomes the Champion of Alola, they receive an invitation from the International Police members' Looker and Anabel. They offer him the task to travel around Alola to hunt and capture the remaining Ultra Beasts.

Pokémon Ultra Sun and Ultra Moon
The roles of the Ultra Beasts have remained unchanged for the most part. However, Lusamine's intentions of capturing the Ultra Beasts are for using them as weapons to protect Alola. Four new Ultra Beasts have also been introduced; Poipole, Naganadel, Stakataka and Blacephalon. The Ultra Beasts are now captured by travelling the Ultra Wormholes and entering in their own perspective home worlds.

Nihilego (UB-01 Symbiont)
Nihilego is the first Ultra Beast encountered in the game. It is a jellyfish-creature with a hollow, clear body. It's said that it's capable of producing a neurotoxin that can affect the mind when possessing its host. It is also reported to be seen behaving like a young girl.

Buzzwole (UB-02 Absorption)
Buzzwole is the second Ultra Beast encountered in Sun and Ultra Sun, exclusively. It is a muscular mosquito-like creature with massive swells around its body. It is said to be so strong that it can destroy a dump truck with one punch.

Pheromosa (UB-02 Beauty)
Pheromosa is the second Ultra Beast encountered in Moon and Ultra Moon, exclusively. It is a tall, thin creature that can move at terrific speed. It is said that it refuses to come in contact with anything, likely due to bacteria and parasites around.

Xurkitree (UB-03 Lightning)
Xurkitree is the third Ultra Beast encountered in the game. It is a tall, cable-like creature that is capable of generating and storing electricity in its body.

Celesteela (UB-04 Blaster)
Celesteela is the fourth Ultra Beast encountered in Moon and Ultra Moon, exclusively. It is a tall, rocket-like creature that is capable of flying and can produce heat that can burn down forests.

Kartana (UB-04 Blade)
Kartana is the fourth Ultra Beast encountered in Sun and Ultra Sun, exlusively. It is a small, yet sharp creature that resembles a paper craft. Its blades are said to be so sharp that they are capable of cutting down steel towers with one slash.

Guzzlord (UB-05 Glutton)
Guzzlord is the fifth Ultra Beast encountered in the game. It is a massive creature with a large mouth with a pair of pincers in its mouth. It has such a large appetite that it has been seen devouring buildings and mountains. Strangely enough, its droppings have never been found.

Poipole (UB Adhesive)
Poipole is a new Ultra Beast introduced in Ultra Sun and Ultra Moon. It is first seen and used by the Ultra Recon Squad in battle. After defeating Necrozma in battle, the player is rewarded with a Poipole of their own.

Naganadel (UB Stinger)
Nagandel is the evolved form of Poipole. Poipole will evolve into this form if it knows Dragon Pulse. It is said that Nagandel is capable of storing poison in its body and fires it out from its needle.

Stakataka (UB Assembly)
Stakataka is an Ultra Beast that appears exclusively in Ultra Moon. It is a large creature that appears to be made up of blocks stacked together.

Blacephalon (UB Burst)
Blacephalon is an Ultra Beast that appears exclusively in Ultra Sun. It is capable of creating explosions in its head.

Trivia

 * The Ultra Beasts are encountered in a similar fashion to the legendary Pokémon, both with unique locations, stats and are genderless. However, whether or not they're legendary Pokémon, or even regular Pokémon for that matter remains unknown.
 * Poipole is the only Ultra Beast that has an evolution.
 * Some Ultra Beasts can be caught more than once. For example, in Sun, you are able to catch more than one Buzzwole.
 * It is said that Lusamine dresses herself and her children after the Ultra Beasts. In Lusmaine's case, she resembles Pheromosa's appearance while her daughter, Lillie, resembles Nihilego.
 * Cosmog is stated to be an Ultra Beast as it is capable of opening Ultra Wormholes. Necrozma is also stated to be one as well, being described as the "reminiscent of the Ultra Beasts". However, the players are unable to capture it with Beast Balls.