Nihilego

Nihilego is is a species of Pokémon in the series of the same name. It is a Rock / Poison Pokémon introduced in the seventh generation in Pokémon Sun and Moon. It is an Ultra Beast with the code name as UB-01 Symbiont.

Main Pokémon games
Nihilego is the first Ultra Beast encountered in Sun and Moon. It appears as a boss in the Aether Foundation Conservatory. Later, Lusamine summons one before going in to its dimension.

In the post-game Ultra Beast quest, Nihilego is the first Ultra Beast that has to be hunted down. It's on Akala Island.