From the description of the official Pokemon site, we can assume Ultra Beasts are neither humans nor Pokémon. That leaves the question, what actually are they? Some people have pointed out the similarities between Lillie and UB-01 but UB-01 is certainly not the only Ultra Beast so would the other Ultra Beasts shapeshift and act as humans too? Maybe UB-01 took the form of Lillie to warn the humans and Pokémon about the other Ultra Beasts? What if Ultra Beasts are like Personas from Persona or Patronuses from Harry Potter? It's crazy because this is the first time in the franchise other creatures besides humans and Pokémon have been introduced and it's totally mysterious and fascinating, I can't stop thinking about the Ultra Beasts now >_>
Perhaps they are man-made mistakes by the Aether Foundation that they are "researching" to try and cover them up for what they have done but to add on the Lillie thing: "UB-01's body is composed of a glass-like substance. However, it's constantly changing shapes, never settling on one. While evidence of something like a survival instinct can be observed in UB-01, no one knows whether it has a will of its own or any emotions. It's said that, for some reason, its movements resemble those of a young girl." -Pokémon Website-
I'm sorry, but the Lillie thing is driving me nuts. It's said in her thingy that she plays an important role in the game. THIS COULD BE IT. Anyway, it'd be awesome (in my opinion) if other people were also Ultra Beasts in disguise. Although there may not be, because UB-01's description thingy specifically states it shapeshifting, but that could also just be because it's the first one to be revealed and they're trying to give out some more information or something. The entire concept is very different, and I'm exciting to see more of this stuff. HYPE! I'm sorry, but the Lillie thing is driving me nuts. It's said in her thingy that she plays an important role in the game. THIS COULD BE IT. Anyway, it'd be awesome (in my opinion) if other people were also Ultra Beasts in disguise. Although there may not be, because UB-01's description thingy specifically states it shapeshifting, but that could also just be because it's the first one to be revealed and they're trying to give out some more information or something. The entire concept is very different, and I'm exciting to see more of this stuff. HYPE!
I feel like Aether Foundation is going to have caused some of this transition with the Ultra Beasts, either making them or trying to build and edit Pokémon thinking they are helping but really causing a mess. The Ultra Beast does of course look like Lillie/Lusamine/Gladion, so maybe a family relation to these Ultra Beasts or that it is really just Lillie. Only time will tell as we see more Ultra Beasts what they really are.
"It appears that the Aether Foundation is also conducting research on these Ultra Beasts." Aether foundation seem pretty sus. Maybe Ultra Beasts were an attempt at man-made people, I mean people make Pokémon so why not? Maybe Ultra beasts are aliens or something, like deoxys minus the Pokémon. The fact that they look like people is kinda creepy though.
It would be pretty great if we did get to catch them in sun and moon and use them for battling but I'm not pretty sure if we'll be able to do that
They are the fathers of all the main Pokémon Trainers (except for Hoenn's). Upon activating my prior knowledge, it is believed that perhaps they are linked to certain experiments (such as Type:Null's). Maybe there is a connection with the Aether foundation? Team Skull? Maybe the Ultra Beasts are otherworldly aliens that the Island Guardians (like Tapu Koko) must face, as seen in the trailer. There could also be a link to Deoxys, because they remind me of a space being.
They're a gimmick that Pokémon doesn't need unless they can/do become legendary Pokémon later. I just don't like them, Pokémon doesn't need big monster bosses that are their own separate thing when the whole game is about monsters, fighting monsters with monsters which you catch and train and use. Keep the monsters to those monsters, these belong in something like Final Fantasy. Feels like they are trying to cram more NEW! things into this gen than they really need to.
I feel like Ultra Beasts are from some alternate dimension that's colliding with the Pokémon world. Maybe the Ultra Beasts are adapting to their environment by changing into humans, similar to the dragons Nils and Ninian in Fire Emblem 7