Aside from their ability to fly, it's not entirely clear how the Winged Monkeys are different from ordinary monkeys. They must be pretty big for monkeys if two of them can carry a child between them. I checked Wikipedia, and it looks like the biggest kind of monkey is the Mandrill, which can weigh about seventy-seven pounds. So maybe they're technically Winged Mandrills, but you'd probably have to ask Professor Wogglebug for the official word on this. Also, I have to wonder if their enslavement to the Golden Cap resulted in increased strength and abilities, as the idea that they could conquer the entire Winkie population seems a bit far-fetched otherwise. Then again, that could just be a result of the Winkies' being unprepared to fight back.
While the Monkeys don't play a significant part in any canonical Oz books other than the first, they do reappear in several later books. dennisanfuso makes them central characters in The Winged Monkeys of Oz. Emerald Wand (for which Winged Monkeys was actually a working title) has the Wicked Witch's niece getting her hands on the Golden Cap and commanding the Monkeys. And while I haven't yet read Chris Dulabone's Marvelous Monkeys or Peter Schulenburg's UnWinged Monkey, I understand that both of them also feature the flying primates.
To finish this post, here are two bits of trivia about the Monkeys in the MGM movie. The first is that the character Nikko, as mentioned in the end credits, is the film's name for their leader. This credit confused me as a kid, because this name isn't actually used in the movie itself. Also, the Golden Cap DOES make a brief appearance, as a deleted scene (I think it was deleted, anyway) had the Witch wanting to use the Cap to have the Monkeys retrieve Dorothy from the poppy field, and the completed film retains a shot of the Witch throwing the Cap away in frustration when the girl escapes. I'm not sure why the Witch would have needed the Cap at this point but not when ordering the simians later on, but there you go.