once could argue that giratinas dimension isnt part of an alernate world, but specifically intertwined with one world. so there should be giratinas for every world that has time and space-dimensions.
so alternate world giratinas (or similar being) should exist for 'ultra-space'.
i mean, overall the ultra beast thing is just a lable put on these surprisingly strong pokemon from other worlds. beause clarly they are pokemon. they use pokemon moves, they listen once you force them into a pokeball (which is known to actually effect a pokemons mind), they perfectly fit within the type-classification of the known pokemon, they evolve like pokemon.
and they can be beat into submission by completely average pokemon too.
they were just different and by the time the next game comes out, a prof will have proven ultrabeasts to be just another type of pokemon. maybe all legendaries actually draw power from ultraspace, which is what makes them stronger than average
Or Ultra Space is another form of space that fills as an in-between of parallel universes.
[drawing power from Ultra Space sounds like the Speed Force, I like that ;P]
Yeah, but post-game we do travel to a parallel universe with day/night swapped (in order to obtain Cosmog),
and everything else we catch there ain't ultra Beasts.
If it didn't originate from Ultra Space, doesn't have Beast Boost, it ain't an Ultra Beast.
It won't be strange for Pokemon-like creatures to exist in an alternate space.
We have Pokemon coming from outer space, laser mutating space viruses into Pokemon, humans that die and become Pokemon.
heck, Pokemon that Rules of Time and Space, an Alpha Pokemon shaping the universe.
In the Pokemon world, Humans are the weird ones.
They even made a movie in-game about Poke Balls catching humans, so it ain't too Farfetch'd to think about creatures in a different realm are Pokemon.
That's how Arceus intended it ;P