The winged monkeys (also referred to as Flying Monkeys in some adaptations) were slaves of the Wicked Witch of the West. They came to be such due to their innate mischievousness and the poor judgment of their King.
To be fair, when the King of the winged monkeys decided to prank a particularly well dressed man by dunking him, fully clothed, into a river, he had no way of knowing that the man's fiancee would turn out to be a (apparently rather touchy) sorceress.
She punished the monkeys by making them slaves to a golden cap. The sorceress only used the cap one time, to order the monkeys to stay away from her and her husband-to-be, but somehow it fell into the hands of the Wicked Witch of the West, who had more ambitious plans in mind for the soaring simians.
In the original story not one of the monkeys were actually named. In comic books like Fables one is named Bufkin.