Thoughtlets: Far Spawn Half-Fiend Analogs

From here

In that case, for what you describe, some terrible analogue of Wilbur Whateley sounds like just the ticket; an unsettling but seemingly human figure who's always completely terrifying when spotted out of the corner of your eye, one whose abominable nature is nearly completely hidden by reality most of the time as though the safe and recognizable was taped over it like wrapping paper... but you can see the seams... and the tape... and the bulges.

You might call it an ectoperan or xenoperan, an ectoxene or edoxene or edoperan. All carry connotations of the outside, the foreign and that which is beyond.

Ectoxene Template