Potential for more Ultra Beasts in future games? Playing through the Rainbow Rocket episode a second time made me realise that Giovanni, who states that he will make an army of Ultra Beasts, also mentions using an Ultra Beast to put Lusamine into a coma of sorts.

However, he never specifies which one. He just says 'That Ultra Beast'.

Nihilego *DID* induce Lusamine into a coma briefly in Sun/Moon, but she doesn't seem to suffer the effects of any Nihilego venom. As for sleep inflicting moves, only Xurkitree can learn a sleep inducing move, Hypnosis. However, given that Giovanni keeps implying that it's the specific power of 'that Ultra Beast', and Xurkitree's main point of interest is it's ability to shoot electricity, which makes me believe there are more undiscovered UBs, just like there's more undiscovered Pokémon, lurking in future games.