I'm not sure. From what I've gathered, the evidence can go either way.
Ingame, they're theorized to be UBs by some characters. Additionally, their stats are all prime numbers, just like the confirmed UBs. Also, they learn all their moves at levels that are prime numbers, which is also just like the UBs.
On the other hand, they don't have Beast Boost as their abilities, they don't have auras like the UBs when encountered, and Beast Balls aren't especially useful for catching them.
All things considered, I personally believe they are UBs, but I won't vehemently defend that, because it's not completely confirmed.