Single post in Alola is coming to PFQ!

The Beast Ball does not always summon Nihilego, but that doesn't matter, as you can trade every egg you get for another egg you want. To explain it, I summoned one of my Beast Balls and got a Nihilego egg, which I traded for an UB-02 Beauty egg. The next time, instead of a Nihilego egg, the Beast Ball summoned an UB-02 Beauty egg, which I was able to trade for an UB-02 Absorption egg. So it does seem like we can trade every UB egg for another. I think that after you get the egg dex, chances of what UB exactly you summon become random (at least at this tier).