>>56077043I like to imagine that Ultra Beasts come from a dead dimension.
Since Pokémon rule the evolutionary ladder, humanity is gone, but the technology remained. After a while, some Pokémon learned how to use it. This created a breach between what is a beast, and what's a "Pokémon".
One war later, now the Ultra Universe is at a zenith: either go and conquer another dimension, or stay, and die as a forgotten pup.
That's why we see humanoid and beast Ultra Beasts.