MitD XII: This Space Intentionally Left Dark

[Kish]

But that would pretty much confirm Protean
You can’t have a mystery if you outright tell what it is
Instead we have been given hints which can fit more than one candidate- which keeps us guessing

That wouldn't actually fit the protean anyway. The protean isn't a single creature, and then another single creature, and then another single creature--it's a boiling mass of features from many different creatures. The closest Xykon or Redcloak could get to that, even theoretically, would be something like "assume a feature that gives you confidence (assuming there is one...) and then spend a move action every round to keep it, such that you can't actually move and attack in the same round."