Right, I don't think "expected behavior" is a good metric for deciding what the MitD is. It does have to be mechanically capable of what we've seen it do, but arguments like Ox's at the end of the last page seem to be arguing from personality:
(as well as an assumption that Xykon is somehow feeding us clues, which I don't think is very well founded.)
And his argument seems to be rather convenient / special pleading to me. When MitD doesn't behave like a Protean, that's evidence he isn't one. But when he doesn't behave like a Hunting Horror, that's evidence Rich is showing us about how he's not typical for his species, and thus actually further proof he is a Hunting Horror.
This is particularly confounded by his mixing in a mechanical element as a personality / behavior element; the Hunting Horror doesn't just "hate the light," it's actively damaged by it. Which gets to my point about mechanics and I think the same one Keltest is making: MitD's personality may not be typical for his species, but as far as we're led to believe, his stats, strengths, and weaknesses are. So something that mechanically should happen for a particular species, and doesn't-- let alone is something Rich knowingly wrote and drew in the exact opposite manner of how it would work-- at least needs an attempt to explain why it doesn't beyond "He's just different." Similarly, while I think the storytelling reasons make the Protean the best choice, they also wouldn't matter if MitD was shown doing things impossible for a Protean and we had already eliminated it mechanically.
(Plus, he's just factually wrong about the alignment, per Tzardok's post. And he's referenced "no alignment" multiple times in the last handful of posts, so he must consider it important to his argument.)