Chaos Beast Monsters are not the same as Chaos Giant Beasts. The latter are everywhere; as long as one orders the Chaos Giant Beast tribes to breed vigorously, countless can be produced in short order.
However, Chaos Beast Monsters are a different story.
Within each of the thousand or so tribes of Chaos Giant Beasts, there exists a King. Only one Chaos Beast Monster is born for every thousand tribes. One can imagine just how slim the chances of their appearance are.
Chapter 351