Ivashu are creatures of Ilvir, known as the Fatherless Multitude by the faithful, or the Accursed Beasts of the Barren Circle by non-believers. Ilvir creates the Ivashu at Araka Kalai, making use of a limited number of souls over and over again. Some Ivashu are fairly common, because they have proved most adaptable to survival, or because they are the easiest to create. However, Ilvir also enjoys experimental lifeforms and can produce any conceivable beast in some quantity. Many varieties are unique, created to perform a specific task, or just to amuse the deity.
Thematically, this means something like a Beholder isn’t one of many, but a unique being. “The Eye-tyrant of the Black Cavern” or somesuch.