Xykon doesn't seem to suspect the monster as the one who stole his kill, so I'm prone to thinking the monster's wish-like ability is not a typical feature of whatever Xykon thinks it is. Are there monsters out there whose appearance is in the eye of the beholder? Redcloak, Xykon, and the hunters to one extent or another seem to think they know what it is, but it is in some other way an unusual specimen of that. Could they all be wrong?