"The Donkey" is a German fairy tale collected by Brothers Grimm compiled in the Grimm's Fairy Tales.
It is Aarne-Thompson type 430, The Donkey Bridegroom.
Synopsis
A king and queen long lamented their childlessness until the queen gave birth to a son who was a donkey. The queen was grieved, but the king had him raised as a prince. He was very fond of music and insisted on learning to play the lute, at which he grew skilled. One day, he saw his own reflection in a pool and grew so disturbed that he wandered the world. He tried to stay at the castle of a king with a single daughter. When they would not let him in, he played outside until the king heard his music and let him in. He insisted that his proper seat was with the king. After a time, he grew sad. The king questioned him about this until he learned that the donkey wished to marry his daughter. The king agreed, they married, and in the night, the king set a servant to watch the couple, to ensure the donkey would behave well. When the donkey went in the bedroom, he took off his donkeyskin and changed into a handsome youth. Even though he put on his skin again in the morning, the daughter assured her father that she was well pleased with her bridegroom. The servant told the king what had happened. The next night, the king stayed up and when the couple were asleep, he burned the donkeyskin. This distressed the donkey, but the king persuaded him to stay by offering him half his kingdom. When the king died, he had the whole kingdom, and when his own father died, he had two kingdoms.