There was something peculiar with the six horned monsters.
Aside from the fact that the five horns could not even begin to compare with the amount of power they had; they were also close to the level of being one of the aids of the [Chaos Monarchs]. A feat that could only be accomplished by creatures which were close to the strength of the [Monarchs] themselves.
This is the reason why there are only a few of them that existed and, the reason why, only a few of them were allowed to exist. After all, the said level of strength also entailed that, they took up too much of the generated [Chaos Energy]. In fact, only a hundred of them would be needed in order to deplete all the [Chaos Energy] that Pier has worked up on creating.