The answer to this can be found at the beginning of Chapter 14.  The chapter is entitled "The Winged Monkeys," which is appropriate, because that is how Dorothy and her friends get back to the Emerald City.

The Winged Monkeys had been controlled by the Wicked Witch of the West.  She controlled them by using the Golden Cap.  After Dorothy killed the Wicked Witch, she took the Cap.  As she and her friends were walking towards the Emerald City, the Queen of the field mice reminded her that she had the Cap and suggested that she use it to get the monkeys to carry her and her friends back to the Emerald City.  Dorothy followed the Cap's instructions, summoned the monkeys, and had them bring the travelers back to the Emerald City.