I suppose that perhaps, as Transfigurating into an Animagus is difficult and dangerous enough, doing it into a magical creature would be the double risky and hard, because these being has their own powers. But yes, I've always wondered that, as well as what happens with Patronuses... Why all the Patronuses forms we've found are normal animals and not a dragon or a .... kelpie?
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I bet you could probably turn into a magical creature (i.e. a unicorn, dragon, etc.) But I wonder if you could really turn yourself into a half-human or human-like magical creature. Since that species (like a merperson or a centaur) is also part human, would it really be possible to transform yourself into that species? I mean, magical creatures in general just seem like it would be harder to do, since they are magical, but still possible. But if it's a humanoid species, I don't know if that would actually be possible. unsure.gif
I think that you couldn't because humanoids aren't animals or just beings, they're actually communities, thinking beings, and so ... you can't just turn into them!