I think they are Pokémon as well. When you look at the description of Type: Null at the official site, it says this: "This Pokémon wearing a mask has been dubbed “Null,” meaning nothing.So Ultra Beasts aren't Pokemon?
The shapes of its front and hind legs are clearly different. The reason is that Type: Null was constructed to synthesize the strengths of various Pokémon, enabling it to adapt to any situation.
The mask fitted to Type: Null’s head is a piece of equipment designed to control its latent powers. It’s extremely heavy, so it also serves to hinder Type: Null’s agility.
To complete a certain mission, there was need of a Pokémon powerful enough to rival those Pokémon often spoken of in mythology."
They are referring to it as a Pokémon and also hint at it that it is man-made or somehow artificially crafted.