<p>Monstrous? Certainly. <i>Dark</i> per se? Not really. And Hei Bai's control over his form was pretty good, if you think about it; he could briefly assume his dark form when he wanted to caution others, something that suggests at least a little focus on his job. Even Wan Shi Tong managed to keep that eerie yet tasteful shape when he first met Jinora. If he'd actually gone dark, like that bunny, wouldn't he have assumed his wyvern form?
</p><p>Frankly, I don't even think Koh <i>can</i> change form; he seems too comfortable the way he is, which is both really creepy and yet somehow soothing. He switches faces to show emotion, he doesn't have to take on a different form. Neither, by the way, does his mother. And even General Old Iron didn't really change either; he just looked a little angrier and only appeared in rainstorms, which might give him a purple halo.
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