Quote self!
People get turned into majins, Shizu would have been one, though what that specifically means or if there is any particular way it is usually done, we don't know.
We do not know exactly what that means as the audience, but the people of the world obviously do. They're heavily associated with demon lords. I would assume the Beastketeers from the first episodes this season were Carrion's majin the way that Myulan is one of Clayman's.
Can it happen to monsters? Probably can, assume so since we've seen a tiger and snake one.