Question, Rito. Rek'Sai is to Xer'Sai, but what are the other Void races called?

Well there are several possibilities

1.They are either called Xer'something too 2.They are called something'something-else 3.They don't even have their own species,Cho'gaths lore mentions how he is a voidborn,a leader among void monsters,Rek'sai might not be a voidborn and simply a very big void monster and the voidborns are maybe unique.

I think Shurimans just named the Xer'Sai, actual void monsters that aren't in valoran are probably unnamed. I think that Vel'Koz, Cho'Gath, Kog'Maw, Kha'Zix, etc are all just names given to them by residents of Valoran.

Using Rek'sai's name as an example, the first part of a Voidborn's names is the species name and the second part is where they entered Runetera. Cho'Gath would literally mean Cho of Gath. There could be separate species where four of the Voidborn champions originate from: the _'Gath, the _'Maw, the _'Zix and the _'Koz.