Introduced in SM
Nihilego
Buzzwole
Pheromosa
Xurkitree
Celesteela
Kartana
Guzzlord
Introduced in USUM
Poipole
Naganadel
Stakataka
Blacephalon
Cosmog, Cosmoem, Solgaleo, Lunala, and Necrozma share similarities with the Ultra Beasts, but aren't considered to fit this category. Necrozma seems to be considered a UB in Pokemon GO — it received the Beast Ball's catch rate boost during Go Fest 2024.
Ultra Beast (Bulbapedia)