If you are unfamiliar with the first Koan, please read about the Gateless Gate here:
From the Yes / No Version
When asked by one student (with no realized nature), the teacher says none. When asked by a student who has realized his nature, the teacher responds yes. In other words, the students asking are the answer. One student had to ask the question, followed by the teacher giving him the answer to the student's own nature. The student wishing to clarify already knew that all have Buddha nature. Each student represents the dog. Buddha nature is not something you can possess. Just this All in All what you are. Becoming denotes realization of this as All in All. Being denotes that you realize your identity, nature and unity as one. For instance, being angry and full of ill will denotes disunity, not Absolute Buddha nature. This makes a changing state of impermanence, which is not being. Unity (one hand clapping) denotes unity of two hands coming together as one. Play two trumpets in tune and you hear one trumpet. Unity of all things as one. All Koans are this answer when identity, nature and unity make one from all the 10,000 things. Your first face before creation is this.
No Only Version
In another answer to MU, the word means does not exist (no such thing as a dog without Buddha Nature). In other words, "Has a dog Buddha nature?" Buddha nature is not a named thing like a dog, therefore a dog is not named by Buddha nature as an individuality. On the other hand, Buddha nature is the Dog. Its predication is Buddha nature one with all in all. Buddha nature is not a dog possessing the nature of a dog alone. A Dog is Buddha nature as a thing. Has a dog Buddha nature? The question is incorrect of course. A dog that is not Buddha nature would be no thing (nothing). MU. Things which are predicated are all the 10,000 things named.
What then is the one unnamed and unmanifest no thing (nothing)? What is the nature of nothing? What cultivates our nature leading to realization of identity and unity with the Buddha, Dharma and Sangha? Identity, nature and unity are all one thing. Buddha nature. Has a Dog Buddha nature? It all depends on how you see nothing. What is being empty of things? No thing at all. This. Buddha (Identity), Dharma (Nature), Sangha (Unity of all Things as One Identity).
Another answer to MU: Same same, only different.
My Answer on Reddit
I recently answered a funny question on Reddit: "Does a chimp have Buddha Nature?"
MU is nothing, or no such thing. No Thing at all. Since all is Buddha Nature, something without (outside of it) is not possible save nothing, which is no thing at all. Go back to the first Koan in the Bluecliff. Emptiness, without holiness. Without (outside of) holiness is emptiness. What, then, is inside holiness?
Go to the Tao 1. What is unmanifest and unnamed? Emptiness. What is the mother of 10,000 things? Holiness. The work done in creation is the relative aspect of the Sangha (Assembly). The basis for this work is the value (Tao 11). All in All. Emptiness is the potential of the whole, which is and must be imperminance to become. Being is unchanging, which is also necessary for Absolute to be with its assembly as an anchor. In Physics, this is invariant symmetry as the anchor for translational symmetry.
The buffalo’s head sprouts horns
As he emerges from the weeds (relatives),
In a dream, he tries to speak
Of the valley of the timeless spring.
Although he has bathed in the fragrant waters,
I hit, saying, “Not Good Enough!
How will you impart strength to others?”
MU - As the three ox states about holiness, once you find the truth, what value will your domesticated ox impart to others? Cultivation of the seed. Enlightenment is only half if absent the work of nature, which is our compassion for others. It's not just for you. But then again, it's all you. Both and none. Raise one to know three. Even a chimp can become a human, rise to heaven, then learn for realization as a Bodhisattva. What is it that a Bodhisattva finds keeping them in the work of their own holiness discovered? Both Nirvana and Samsara are the same if you arrive at who you are, your nature and your unity. Identity, Nature and Unity bring Nirvana to Samsara. One. There is only one thing in creation worth being. By removing all relative things (images of yourself), what is left?
But he asks, "How will you carry it out?" One realization answers all Koans. Who are you? What is your nature? Who are others in relation? All the same question.