Cath Palug is a creature that is essentially born to the world itself, I'm guessing it's function is to control rampant humanity population, akin to a white blood cell, which makes sense given that Primate Murder's other title, is The White Beast of Gaia. Would also explain why it would be able to be kept under control by Altrouge Brunestud, considering how close she is to Arcuied Brunestud(Being her sister and all) whose other self is Archetype Earth. Now a quick bit to point out, Primate Murder is not Beast 4, but Beast 4 is Primate Murder, meaning that Primate Murder at this stage is a step before becoming 4, just CCC Kiara isn't Beast 3R, but FGO Kiara became Beast 3R
Basically Cath Palug/Primate Murder is akin to a white blood cell and humans are the contagion to get back under control, however, White Blood Cells can encounter glitches that make them not function as they should, say mutating from it's designed role for example by perhaps taking in TOO much of said contagion quicker than it can break it down, basically is what Cath Palug/Primate Murder becoming Beast IV is when it reaches critical levels of Regret and Envy consumption.
So what if in this case, Fairies AND Humans were seen as contagions by Cath Palug/Primate Murder? Or became too blurred together as threats to the earth? Now the other bit is that LB Cath Palug additionally has no connection to Will Gaia, but still has the role of POPULATION CONTROL, what is currently in high amounts of Population right now and are mimicking humanity civilizations and culture without fully understanding things? That would entail that the line of distinction between humans and fae could be more blurred from a simple thinking entity whose singular role is population control and white blood cell of the planet. Simply on the notion that the Fae do not comprehend that their acts of whimsy might be leading them to be marked as contagions because they are copying humanity in both the positives AND negatives by mimicking the way they live and such without understanding the that by doing so, they are essentially blurring the line between humans and fae from a simpler more primal creature perspective, they would have a more similar "scent" to one another possibly."Gaia protects all creatures on Earth by using Nature Spirits and creatures of the Earth, but as mankind becomes independent, Gaia increasingly perceives humans as a source of threat due to their reckless destruction of the environment.
Gaia's response to the human threat is producing beings that can cull or rule mankind. Primate Murder was created to quickly eliminate humans."