RedJirachi
Veteran member
It's definitely a UB in the same way that Cosmog's family are: they came through the worm hole ages ago. The fact that the BSTs are prime numbers and that they all share the same ending in their cy is proof enough that they're UB even if the games themselves are trying to go "what no, thats silly, get out of here"
It's not a UB in the same way Kyurem came from a meteor, to put it another way.
We don't know enough about the Ultra Beasts to be sure that Necrozma must exactly be one. Though it is possible it was from Ultra Space, but wasn't a Pokemon yet. Just a strange prism-shaped asteroid that emerged and was crafted by the natives into Necrozma