Ultra Beasts are coming to Pokemon Go at some point in the future, according to this teaser video released by Niantic.
Introduced in Gen 7, Nihilego is an Ultra Beast that resembles a jellyfish and is one of 11 currently known Ultra Beasts part of Pokemon lore.
Ultra Beasts are a group of extradimensional Pokemon that come from space and appear via Ultra Wormholes in Alola. They are rather combative and drawn to those who have also passed through an Ultra Wormhole, due to residual energy.