So the Wicked Witch took the Golden Cap from her cupboard and placed it upon her head. Then she stood upon her left foot and said slowly:
"Ep-pe, pep-pe, kak-ke!"
Next she stood upon her right foot and said:
"Hil-lo, hol-lo, hel-lo!"
After this she stood upon both feet and cried in a loud voice:
"Ziz-zy, zuz-zy, zik!"
Thanks to the famous MGM film, the Winged Monkeys of The Wonderful Wizard of Oz have become nightmare fuel for children and given birth to many horrific reinterpretations.
Which is very strange, because that's not how they were originally intended at all.
The Winged Monkeys appear in only one of the Famous Forty Oz books: The Wonderful Wizard of Oz. They get a mention in the next book, but we'll get to that.
The Wonderful Wizard of Oz implies that Quelala used the Golden Cap once to tell the Monkeys to stay away from Gayelette as she didn't like to look at them, a service they were only too happy to render.
Somehow, the Golden Cap fell into the hands of the Wicked Witch of the West, who used it to make the Winkies her slaves and drive the Wizard out of the Winkie Country.
Her final use of the Golden Cap was a last ditch effort to destroy Dorothy and her friends, except the Lion, who would be taken to the Wicked Witch's castle to be made to work like a horse.
The Monkeys were able to do this, pulling out the Scarecrow's straw and hanging his clothes in a tall tree, and dropping the Tin Woodman onto sharp rocks where he became battered and dented. They were also able to subdue the Cowardly Lion and tie him up and carry him to the Witch's castle.
Dorothy, however, they would not harm because of the mark of the Good Witch on her forehead. They recognized it as "protected by the Power of Good, and that is greater than the Power of Evil." They instead took Dorothy and Toto to the Witch, explaining their case before leaving.
After the Wicked Witch's demise, Dorothy happened to find the Golden Cap and was later advised by the Queen of the Field Mice to use it to summon them to take her back to the Emerald City.
Dorothy's last use is to have them carry her and her friends over the Hill of the Hammerheads and into the domain of Glinda. (Some adaptations shorten this easily by having them take them straight to Glinda's palace.)
Dorothy turns the Golden Cap over to Glinda in return for instructions on how to use the Silver Shoes. Glinda says that she'll use it to send the Scarecrow back to the Emerald City, the Tin Woodman to the Winkie Country and the Lion to the forest where he defeated a giant spider. She says that she will give the King of the Winged Monkeys the Golden Cap, ensuring their freedom forever.
Or did she? In The Marvelous Land of Oz, the Scarecrow mentions that Glinda now commands the services of the Winged Monkeys. What?
Perhaps the Scarecrow's mistaken. Or maybe they pledged their services willingly after Glinda freed them. Or maybe after Dorothy went home to Kansas, Glinda realized she could just summon the Winged Monkeys once and tell them to take Dorothy's friends to their homes. Or maybe she even realized that she could have a servant make the wishes and then keep the cap herself, to keep it from falling into evil hands again.
Maybe that's the story behind Glinda's odd crown that Neill drew her with... It's a modified Golden Cap!
There is a point I did realize about the Winged Monkeys. Apparently, only adult male Winged Monkeys were summoned by the cap. The female and child Monkeys are never mentioned, yet they must exist. And considering that the Winged Monkeys are often summoned to perform tasks of strength, children would be excused from such tasks anyway. As for women, while capable of completing these tasks, it seems likely that they were excused in case any or all serving Monkeys would be injured critically. (Someone needs to care for the young. Remember that Gayelette made the charm and likely did not have an advanced way of thinking.) And likely elderly Monkeys are also excused.
The Golden Cap commands the services of the Winged Monkeys, but when the charm is used, do the Monkeys have to fly all the way there or does the cap make them appear? Baum mentions that when the Wicked Witch uses the Cap that "the sky was darkened, and a low rumbling sound was heard in the air." This is not repeated when Dorothy uses the Cap, however.
I would go with the idea that the Cap makes the Monkeys appear close by the summoner. The extra effects of their summoning reflect the nature of the summoner. The Wicked Witch was evil, so her use of the Cap appeared ominous, while Dorothy's use of the Cap was only for innocent tasks. Who knows what happened when Quelala and (presumably) Glinda used the cap?
Extra bit of trivia: in Alexander Volkov's The Wizard of the Emerald City, Elli (his rewrite of Dorothy) and her friends realize that they could use the Golden Cap a dozen times between them. (Totoshka reminds them that the number should be fifteen.) No more than Elli's three uses actually happen, however.