Day of the Chicago Rose, 11th of Summer, 525 M.E. (Atlas): Gudo Nishijima, for one, offers "ghost of a wild fox" as an alternate translation of Dogen's Yako-zei (spirit of a wild fox). He notes that the term usually suggests criticism of a person who is too mystical and not practical enough, although in this case Dogen is suggesting the presence of something both natural and mystical. 

To add a little more to the full koan of Baizhang's Fox:

Whenever Baizhang gave a talk, there was always an old man listening in the back of the room, but as soon as Baizhang stopped talking, he always vanished. One day, though, Baizhang caught up with him and asked who he was. 

The old man said, “It’s true, I am not a human being. Many, many years ago, I was a priest living on this mountain. A student asked me, ‘Is an enlightened person subject to cause and effect or not?’ I replied, ‘An enlightened person does not fall into cause and effect.’ Because of this, I have been reborn as a fox for five hundred lives."

"Now I beg you," the old man said, "release me from this wild fox’s body. Is an enlightened person subject to cause and effect or not?”

Baizhang said, “You don’t ignore cause and effect.”

At these words, the old man was deeply enlightened. He bowed and said, “I’ve been released from my wild fox’s body. That body is on the other side of this mountain. I ask that you kindly perform for me the funeral for a priest.”

Baizhang had a monk strike the gavel and announce to the community that after the meal there would be a funeral for a priest. Everyone wondered about this because they were all healthy and no one was sick in the infirmary. After the meal, Baizhang led the assembly to the foot of a cliff on the other side of the mountain. He used his staff to poke out a dead fox, then he cremated the body according to the rules.

Almost every commentary I find on this koan acknowledges that the story is of course complete nonsense. Baizhang, they all note, most likely was out on a walk one day and came across the carcass of a fox. Deciding to use his find as a teaching device for his students, he cooked up the whole story about the shape-shifting old man/wild fox.

It's seems fitting that he would want to make pedantic use of a dead fox since his own enlightenment story, according to tradition, involves wildlife. One day, he was out walking with his teacher, Mazu, when they spotted some wild ducks flying overhead. Mazu asked, "What are they?" and Baizhang answered,. "M R ducks." Mazu said, "M R not ducks." Baizhang said, "O S A R. C M Wangs?" Mazu acknowledged,  "L I B. M R Ducks!"

(Snort.) Just kidding. An old Southern joke that I couldn't resist. The story actually goes that when Baizhang confirmed that it was wild ducks that were flying overhead, Mazu asked him, "Where are they going to?" Biazhang answered, "They are just flying." Mazu then tweaked Baizhang's nose (really, no joke, that's how the Chinese Zen teachers acted back then) and as Baizhang cried out in pain, Mazu asked, "Where have they gone to?"

Mazu's punishment was because of Baizhang's Yako-zei answer -  too mystical and not practical enough. In the Zen sense, Baizhang was correct - in the here and now, the ducks were just flying. They did not appear from anywhere and they do not vanish. They are simple flying here and now. But only a Zen student talking to his teacher would answer like that, and for that he got his nose tweaked. O, rocks!, Tell us in plain words. "M R ducks, Baizhang!" 

The post script to this story is the next day when Mazu took his seat to deliver a talk to the monks, Baizhang rolled up his mat and walked out. When Mazu later asked Baizhang why he left, Baizhang said, "Yesterday, you tweaked my nose and it hurt." Mazu asked, "Where was your mind yesterday?'

Baizhang ansewred, "My nose doesn't hurt today."