UB's are interesting to say the least.
UB's are version exclusive in terms of the forms they take on.
Story-wise, UBs come into contact with a human, then take on their characteristics to survive in our dimension. For example, UB-1 met Lillie before the story began.
The people that some of the UB's encounter changes based on version.
UB-1 Lillie in both versions
UB-2 Lusamine in Moon / Gladion in Sun
UB-3 (Bamboo/Origami) Faba/ Plumeria
UB-4 Guzma in both versions
UB-5 (Fat one) Hala in both versions (Somewhat Final Boss)
UB-R (Edgemon) Post-game villain dealing with Looker quest.

You can catch them.

Peace.