I’d like to propose that the Monster in the Dark is a Therblewurkersaurus. A Therblewurkersaurus is just like a Protean, except it doesn’t have any Protean abilities that would embarrass it.

It can’t change its size, so it’s stuck at super strength for the big hit.
It doesn’t change shape against its will, so it can have fixed eyes, mouth, and other appendages.
It doesn’t have +59 Bluff, so it can struggle to Bluff Xykon away from Girard’s Gate
It doesn’t have dimension door, ethereal jaunt, or knock, so it’s helpless to acquire the steak in the jungle scene.
It doesn’t have 39 SR, Blinding Speed, and Destabilize Form, and it can’t manifest Pounce, so it can’t go 100ft, pounce for a full attack, hit with 5 slam attacks, and literally liquefy Redcloak in a single round, with Redcloak not being able to pierce its SR with any of his spells, which explains why Redcloak is not frightened of it.
Also, it doesn’t have +32 Will save to go with that 39 SR, so Xykon can land a mid-level mind-affecting spell on it without comment.
Also also, because it doesn’t have Destabilize Form, Xykon has to order it to swallow Redcloak whole instead of just liquefying him.