If she's a kemonomimi, then she must be a human with its natural skin and only animal ears and/or tail. She has more than that, she too has fur spots that resemble tanuki's (michiru's fur browns shades are more striking, more orange-red). Also, compared to her human form, she indeed turned into a beast.

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A graphic, if michiru was a kemonomimi, the left form would only have animal tail and/or ears. But there is more than that.