Personally, I think Protean doesn't fit, because the Proteans' main gimmick is their shapechanging abilities.

My reasoning: the only reason Xykon makes the Monster stays in the Darkness is because he wants a big, dramatic reveal during a worthwhile encounter against heroes.

In other words, the sight of the MitD/people reacting to that sight while in a combat encounter is something Xykon would deem big and dramatic enough to set up in advance and keep ongoing for months and years.

Now, there can be many reasons for that... but the fact is that a Protean can look like most creatures in creation. So if Xykon wanted a big, dramatic reveal would be for the Monster first look like an harmless bunny or a puppy or the like, then BAM! turn into a big dangerous being like a Gray Render, revealing his shapechanging power.

What I mean is that visually obscuring a being who can look like any creature isn't a great way to conceal them, in-universe-drama-wise.

Unless Xykon is trying to set up a fake-out by having the Monster reveals himself as an harmless being... and then change into the scary form. But I doubt Xykon would trust the MitD with that kind of script.