On the whole matter about the Ultra Beasts... a question pops into my head: What makes a Pokemon? What makes the Porygon line, an artificially-made living computer, a Pokemon and what does not make the Ultra Beasts one? I mean, being alien is not exactly a point given the Clefairy line is given minor hints that they are aliens, and Deoxys is one, yet they are Pokemon.
Hope a question like that is answered in-game.
Man, I was wondering about it, and the answer is totally unclear. There are Pokémon made from all kinds of things. There are Alien Pokémon, dead Pokémon, toy Pokémon, digital Pokémon, god Pokémon.
I dare say all creatures from this dimension are Pokémon. Yeah, Giratina comes from another dimension, but it isn't originally from there. It was banned by Arceus.
I have a theory that may answer this. Maybe humans are also Pokémon, but they evolved into a different kind of creature. All Pokémon seem to be able to understand each other, even though they speak "different" languages. Maybe a specific group of Pokémon were highly intelligent and developed differently. As time went by, this kind of Pokémon, now known as "humans" became the dominant species in the Pokémon world, and their higher intelligence made them create devices that allowed them to enslave the other creatures. Eventually, the devices became less a slavery thing and more of a bonding thing that helped the "lesser" Pokémon to reach further heights in power, making them evolve into different things.
Also, the "human-pokémon" were able to use these devices to transform the other Pokémon into bio-digital creatures, live creatures that are able to be uploaded, downloaded and transported as data. When they did this, the brain of the "animal-pokémon" was transformed in a way that store bits and bytes and have limited capacity. That's why they can only know four moves at once, having to forget one if they want to learn another, just like you have to delete files from a full drive in order to put new ones.
The same way all creatures on Earth developed from the same basic cells, and we are all basically the same in the molecular level, there can be an argument for this.
Also, there are hints in the games and other canon things that humans and pokémon used to have relationships. The existence of humanoid Pokémon can seem as another hint to this. They might have stopped in the evolutionary chain in their way to become humans. I could develop even further, but I think you get the gist of it.
UB are from a completely different dimension/universe, so they don't have the same cells that make a Pokémon. Therefore, they are really alien creatures in this universe as a whole. They make no sense to us, and we don't make any sense to them, so, when they come around, they try to take the form of a human in order to be understood by us. Or that is the form our brain recognizes them, as their real nature is too alien for our minds to grasp, so we see them in a different way than they really are.