New Pokémon games generally bring with them new Pokémon. Ultra Sun and Ultra Moon don’t begin a new Generation, however, so don’t expect a hundred or so new creatures to catch, but there a few new ones, as well as new forms for old ones, that weren’t in the original Sun and Moon.
Most of the new Pokémon in Ultra Sun and Ultra Moon are new Ultra Beasts. Poipole is a new, Poison-Type Ultra Beast that can evolve into the new Naganadel, a Poison/Dragon-Type, upon leveling up while knowing the move Dragon Pulse. Stakataka is a Rock/Steel-Type Ultra Beast that is exclusive to Ultra Moon and Blacephalon is a Fire/Ghost-Type Ultra Beast exclusive to Ultra Sun. Other than these Ultra Beasts, there is only one new Pokémon in these games, which is the Electric-Type Legendary Zeraora. Zeraora is currently unavailable in the games, however, but is expected to be distributed via event sometime in the future.
There are also some new forms for a couple of returning Pokémon. Lycanroc has a new Dusk Form in these games, which can be achieved by evolving a Rockruff with the Own Tempo ability. This Dusk Form is pretty similar to Lycanroc’s Midday Form but with a generally slightly higher Attack stat and slightly lower Speed stat. Also, Dusk Form Lycanroc learns Thrash upon evolving, instead of Accelerock or Counter.
Necrozma has three new forms in these games: Dusk Mane Form, Dawn Wings Form and Ultra Form. Dusk Mane Necrozma can be attained by fusing Necrozma and Solgaleo. When in this form, Necrozma can learn Sunsteel Strike and has stats resembling Solgaleo’s, but with a base total of 680 instead of 600. Similarly, Dawn Wings Necrozma can be attained by fusing Necrozma and Lunala, can learn Moongeist Beam and has stats resembling Lunala’s, but also with a base total of 680 instead of 600. Necrozma also takes on the typings of Solgaleo or Lunala upon achieving its Dusk Mane or Dawn Wings Forms. Ultra Necrozma can be attained by having Necrozma hold Ultranecrozium Z and using Ultra Burst in battle when in its Dusk Mane or Dawn Wings Forms. Ultra Necrozma is a Psychic/Dragon-Type and has a base stat total of 754, making it one of the strongest Pokémon in the game. Ultra Necrozma also has the new ability Neuroforce, which makes super effective moves even stronger.