I think the (bestial)Illborn are the byproduct of the harvesting, and corruption of the soul of a creature.
The term Illborn most likely applies to all creatures who have their soul ripped out, but lack the Star.
While in the process of harvesting the Soul star(or invalid method of reviving a being), they might corrupt the soul itself, malforming the disconnected body and mind of the creature it belongs to.
Like how when Rilitia died, and was reborn, it was hinted it had to do with the creation of Illborn. Rilitia's soul star was heavily corrupted and along with it, her mind and body were corrupted as well, turning her into a giant triangle worm with wings that had the one and only intent to find Ciro. The strength of a soul might also co-relate to the strength of the Corrupted Illborn, as a sibling, Rilitia had a relatively strong soul, this led to completely destructive and almost overpowered Illborn with the strength to possibly destroy the Warrens themselves.
Her corrupted soul might be a lead in the Illborn theory, as because a Soul is something that contains the information of what the person is, and what they have done
A brief light shone in front of him. It was beautiful, simplistic. It was Slog. It was everything Slog was! Every emotion he'd ever felt, every blorb he'd ever orbled, every adolescent Red Slime to ever turn him down. Slog smiled for a second, and it was gone.
corrupting the information destroys thought processes, and changes form, essentially changing what the person thought they were and what they have done
The highly corrupted form targets those who are not Illborn, or those who get in it's way, as said by Al, who said that the monsters never attacked Wilford, and were attracted to him greatly.
but you've always known there was something different about them. For example: they only attack specific people - like you - as if they knew to hunt them down. You try not to think really hard about things like this. Ruins your calm, you know?