That would be an interesting idea, that Eternatus was an Ultra Beast or maybe another Necrozma. The question is, if these two's genotypes did indeed match...what spawned them? Spawned as in family and genetics and all that. That would be an interesting idea to expand on.

^Whatever did it, might be connected to Area Zero crater. They both deal with meteors and space time manipulation (Ultra Wormholes and Paradox Pokemon )

But Necrozma isn't even necessarily from a defined dimension and seems to come from Ultra Space itself, while Eternatus is from the main world.

Adding onto what’s talking Gina said maybe since necrozmas from an entirely different world, maybe it is the parallel of eternatis in their world, and solgaleo and lunala had to enlist the help of the protagonist to help set it right and they broke ultra space to fix it

I’ve been wanting the 8th generation Legendaries to get Z moves in future games (especially Eternatus and Calyrex)