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None, a Shih-tzu is a Shih-tzu. But it can be mixed with other dogs.
The Shih Tzu
The correct spelling is "Shih Tzu".
The plural of Shih Tzu is "Shih Tzu". "Shih Tzu" was originally a Chinese word and Chinese words do not have separate singular and plural forms.
Yes, Shih Tzu has an undercoat.