Nihilego

Nihilego is a Rock, Poison-type Ultra Beast from the Alola region. It does not evolve into or from any other Pokémon. Its codename is UB-01 Symbiont.

Trivia

 * Nihilego is the only Ultra Beast that could be caught in a Poké Ball but not a Beast Ball as Beasts Balls were not yet introduced in the game at the time of its debut.