Okay, UB-02 Beauty's similarity to Lusamine has made me significantly more amenable to the idea that the Ultra Beasts are transformed humans. On the other hand, I still have significant doubts. A large part of this is that Beauty is apparently a Moon exclusive, and I don't think that the transformed version of a character as important to the plot as Lusamine is shaping up to be would be a version-exclusive.
On the other other hand, I don't think it's so far-fetched anymore that the UBs may have connections to humans. I can't deny the 02 Beauty/Lusamine resemblance, and now that I've thought about it more, 01 does resemble Lillie more than I initially thought (though I maintain that it looks more like a jellyfish than a girl). I don't know what the connection might be, but it's just as likely as not that there is one. Of course, Game Freak likes their red herrings, so...
Also, I agree that Ultra Beasts should not be catchable. Some of Pokémon's mystique vanishes when you consider that Arceus, the creator of the universe, can be captured and commanded by a ten/twelve-year-old kid. True, there have been good arguments against this viewpoint on this thread, but that doesn't change the surface picture. Ultra Beasts look like they're going to be bringing back that mystique, and that'd be diminished if they're catchable. And in any case, it's practically been confirmed that UBs aren't Pokémon, and if they're not Pokémon, why should they be catchable?