Apparently the luckless donkey has lost his tail, and is asking his doctor how he intends to fix it.
Are you aware of a popular children's party game called "Pin the Tail on the Donkey"? A picture of a donkey is placed on the wall, with his tail missing. Children are blindfolded, one by one, and given a piece of string (the tail) with a pin in it, and gently directed toward the picture. The one who pins the tail closest to the correct place wins.
It is unrelated to your question in another thread about why donkeys are considered stupid. That concept is not expressed here.