A monk asked Master Chao-chou, "Has a dog the Buddha Nature or not?" Chao-chou said, "Mu!"
n this bit of dialogue, the student is asking about Buddha-nature. In Mahayana Buddhism, Buddha-nature is the fundamental nature of all beings.
In Buddhism, "all beings" really does mean "all beings," not just "all humans."
And a dog certainly is a "being."
The obvious answer to the monk's question "does a dog have Buddha-nature," is yes.
But Chao-chou said, Mu. No. What's going on here?
The fundamental question in this koan is about the nature of existence. The monk's question came from a fragmented, one-sided perception of existence. Master Chao-chou used Mu as a hammer to break up the monk's conventional thinking.