If this blog's user is still active... thoughts on the conciet of heroic-aligned magician figures whomst are effectively borne of demons, animals, or monsters? Merlin's father being an incubus in some tellings, the legends that Abe no Seimei's mother was a fox-wife, etc. What is the relationship between wise-men, shamans, wizards, etc and monsters?
This is interesting, because it depends heavily on your working definition of monstrosity, and it's also incredibly intersectional in terms of research areas. Though quite luckily, my supervisor actually specialises in medievalism and fantasy, so we've discussed some of this sort of thing.
Personally, in my work, my definition of monstrosity always requires the presence of the supernatural. But equally so, a core tenant of monster theory is the dynamic of the norm/the other. That's what the monster is: the embodiment of that which a society has deemed 'other', wrong, outcast.
A concept I've been writing on for the past few months in my thesis relates to the perception of monstrosity; is a monster only a monster because we perceive it to be one? That's a very long conversation, but the perception of monstrosity is key. So in a sense, it depends on both the author intention and the audience understanding of a particular character in a work.
There's a thin line between characters who may have supernatural abilities or origins and monsters; and that comes down to whether or not they are presented to the audience as a monster.
A good example of this would be...Damien from The Omen vs Adam Young in Good Omens.
The narrative of The Omen presents Damien to us as a monster. He's devil spawn, he's the antichrist, he's a monster. We go into that with the preconceived notion of him as a monster, even before the character actually does anything 'evil'.
Adam Young in Good Omens is a direct parallel to Damien. He is, fundamentally, the same character. He's the antichrist, he's the spawn of Satan. But Adam isn't presented by the author and to the audience as a monster. Even though he is, objectively, the antichrist, literally the spawn of the devil.
All of this to say that in essence, with folkloric characters like Merlin especially, it depends on the individual depiction of the character. For instance, while some versions of Merlin depict him as being half-demon, others claim he was an immaculate conception ala Jesus Christ.
Personally, I think there's a fair few depictions of him as a character that someone could argue as being monstrous. In fact I would go so far as to say most; because despite his role as advisor to the king, his inherent 'supernatural' abilities position him as 'other' in his society. The same could probably be argued for a lot of those kinds of wizards, sages, magic users and so forth--because inherently their abilities place them external to the norm of their society.
I don't know if I actually answered your question but I hope it helped a little!