It's a manufactured pokemon, that in canon, was unstable to the extreme and ended up only being "tamed" by a very bold but empathetic young man, and the genius young trainer of the player character. It's also a manufactured pokemon that was clearly intended to be comparable to Arceus itself, thus putting it in a class much, much more akin to Mewtwo than to an intentionally mass-produced thing like porygon.

Heck, for that matter, the games make it clear that there's no shortage of Ultra Beasts back in their home dimension, but I don't think anyone would argue that they aren't intended to be rare pokemon on a level similar to legendaries and mythicals. Type: Null is essentially in that same class game mechanics and lore-wise.