Based on your answer to the Ultra beast question, WHAT IS A POKÉMON??
Well, I think the Ultra Beasts (and Deoxys, for that matter) suggest that it’s not actually a biological distinction, or even anything scientific at all. I think they just call something a Pokémon if it can battle and take orders. Pokémon moves and types are categories invented by humans to describe what different creatures can do; they don’t necessarily indicate anything consistent about physiology. So it’s not clear whether Ultra Beasts are Pokémon or not, until someone catches one and uses it in a battle, and that fact is formally recognised by one or more Pokémon Leagues; after that, they definitely are. I suspect it’s almost more of a legal/rules issue than anything else.