Another tale from the Arabian Nights collection - this is an excerpt from "Julnar the Sea-born and Her Son King Badr Basim of Persia."
Badr Basim, after many adventures (including being turned into a bird by the woman he had hoped to marry) finds himself on a sea voyage. His ship is wrecked in a storm, and he is lucky to escape.
Clinging to some debris, he comes to shore at the gates of a great white city. Attempting to enter, he can see no people but he is waylaid by a great number of horses, mules and asses. Fleeing, he returns to the water and swims around to the back of the city, where he climbs over the wall.
The city, it seems, is virtually deserted except for the animals, but Badr Basim comes across a pious old man named Abdallah. Abdullah tells Badr Basim that this is the City of the Magicians, and it is ruled by Queen Lab. Every forty days, the Queen takes a young man for a lover, and at the end of that time, transforms him into a horse, a mule, or a donkey. It seems the queen respects Abdullah and lets him live in peace because he is a faithful servant of Allah. Abdullah tells Badr Basim that he will pretend the prince is his nephew.
By and by, the Queen visits Abdullah, and is introduced to Badr Basim. She invites him back to the palace. The old man makes her promise she will not transform him and she agrees. She takes the prince back to her palace and pampers him for the rest of the day. The next morning he awakes alone and after searching the palace, sees a white bird playing with a black bird. The white bird shimmers and becomes Queen Lab.
Badr Basim pretends he hasn't seen this, but finds himself feeling jealous: the black bird must be another of Lab's transformed lovers. He asks to return to Abdullah and is allowed to do so. Abdullah confirms the prince's suspicions and says Lab will not keep her promise not to enchant him. She will offer Badr Basim parched grain to eat. He must pretend to do this, but if a grain actually passes his lips, she will be able to turn him into anything she likes. He gives Badr some enchanted grain of his own and tells him when she eats it he must throw water in her face saying “Quit this human form” and choose a shape of his desire.
Sure enough, the Queen soon returns and invites the prince back to the palace where she offers him parched grain. He pretends to eat it but nothing happens to him. She is puzzled but jests with him and he tricks her into eating the grain he brought with him. Then he does as the old man instructed him and orders Queen Lab, "Quite this human form, and become a she-mule!"
Lab immediately falls to all fours, her body rapidly growing and bursting from her clothing until she stands before him as a comely white she-mule. Abdullah gives him a bridle and tells him to lead her from the city, which the prince does. After three days however, they are met on the road by an old woman who tells him she wishes to buy the mule from him. Badr refuses, and when she insists, names an impossibly high price. To his amazement, she offers him the sum immediately, and when he tries to tell her he was joking, advises him that in this land, people are put to death for lying.
Reluctantly, Badr Basim hands over the white mule. The old woman is Lab's mother, and promptly restores the she-mule to her human form, before they fly away on a conjured Ifrit.