From Bulbapedia, the community-driven Pokémon encyclopedia.
An example of an Ultra Beast Pokémon
Ultra Beasts (Japanese: ウルトラビースト Ultra Beast) are a type of Pokémon in the Pokémon Trading Card Game. They were introduced in the Crimson Invasion expansion and include all classified Ultra Beasts from the core series plus Necrozma's alternate forms (Dusk Mane, Dawn Wings and Ultra Necrozma). Certain attacks, Abilities, Trainer cards, and Special Energy cards interact specifically with Ultra Beast cards.
Every Ultra Beast card has an attack or Ability that affects or is affected by the player and/or opponent's Prize cards. In the case of an Ultra Beast GX card, this is always its GX attack. Several of the Ultra Beast-related Trainer cards interact with Prize cards as well. This association with Prize cards may be a reference to the Ultra Beasts' Ability Beast Boost in the core series, as both Beast Boost and Prize cards center around knocking out the opponent's Pokémon.
Ultra Beast cards are distinguished by a red Ultra Beast label superimposed on the card art below the HP and type. Ultra Beast GX cards have a red graphic instead of the blue graphic depicted on other Pokémon-GX cards. Additionally, the GX attack bar, effect text, Pokémon-GX rule box, and outlines for the Full Art graphics are colored red rather than blue. This red motif may be a reference to the red aura surrounding wild Ultra Beasts in the core series of games.
The Crimson Invasion through the Lost Thunder expansion's subthemes involved Ultra Beasts arriving in Alola and other regions.
Pokémon that have appeared as Ultra Beasts