What would be the evolutionary setup for flying monkeys to evolve. They should have branched of from the same ancestors as present day monkeys.They need to be migratory like modern day migrating birds and should be able to fly large distance.
Talking about flying creatures, there are two types of them: gliders and true fliers.
While it is (relatively) easy for mammals to produce gliders through evolution, true fliers are almost impossible to evolve in mammalian evolutionary history. I explain both aspects of my statement below.
These include creatures which have the ability to expand their skin (usually around the chest and belly) and create a large surface area, which enables them to cover distances much longer than a normal jump. Flying squirrels are a good example of such creatures.
While the how about the emergence of flying monkeys can be answered with some research and educated guessswork, the why part is impossible to answer. As in, it is (relatively) easy to explain which evolutionary steps would be involved in the origin of flying monkeys, it is impossible to explain why those changes would be triggered in the first place.
Gliding mammals do it with an elongated membrane which they stretch after jumping from height. The membrane (in case of flying squirrels) extents from their wrists to toes and provides a much larger surface area for their skins than normal, allowing them to parachute themselves to great distances. Gliding monkeys would evolve with the same setup; an extended skin membrane. The tail might also develop a thick coat of fur to help stabilize them in-flight and allow them to not only change direction mid-flight, but also decrease their speed before landing on their destination branch.
Molecular study of flying squirrels shows that they originated 18-20 million years ago from normal tree squirrels. If you cut the time, you could have your flying monkeys evolving in more or less 8 million years from normal tree dwelling monkeys.
This is almost impossible to evolve in mammals. By true flight we mean powered flight, using wing movement. The main problem with this is feathers. Mammals don't have any form of feathers. In dinosaurs (which led to birds) feathers have a very long evolutionary history before they culminated in flight feather of microraptor, sinornithosaurus and epidexipteryx. The evolutionary process involves several very intricate changes and spans at least 25 million years.
It would be nearly impossible for mammalian evolutionary process to go all the same way as dinosaurs, first producing insulating feathers, then gliding feathers and finally flight feathers. Then there is the problem of arm size and strength. Monkeys at all do not have the arm length ratio of those little dinosaurs with long arms, meaning that even if flight feathers somehow did originate on monkey arms, they would be unable to fly due to their small sized arms. Then there is the issue of muscle strength. With time, the dinosaurian lineage leading to birds, had more and more powerful muscles in their shoulders, finally allowing them powered flight. Furthermore, the shoulder joint structure of those dinosaurs was much different than modern monkeys, further complicating the process. Then there is the issue of mass. The dinosaurs originating into birds were very small and lightweight. Monkeys today are quite heavy in comparison. If those problems were not unsurmountable in their own right, there is the issue of furcula, or the wishbone, which is present in the chest of birds and is important in offering a central point for muscle attachment and maintaining the shape of the chest. No mammal has a furcula. And to hammer the last nail in the coffin of mammalian powered flight, there is the issue of air-sacs, which all birds and dinosaurs possessed, but no mammal has it. Air-sacs are an extremely efficient method of providing a constant flow of fresh air to the lungs. It is required for high energy activities such as flight. Without air-sacs, a mammal would only be able to fly a couple hundred meters (at most) before exhausting and panting for the next two-three minutes.
In short, forget powered flight for mammals. There are just far too many problems to solve and a nearly endless list of complexities involved. If you want to have flying monkeys, make them gliders.
The primary problem with the origin of flying monkeys is, why would they evolve in the first place? As in, what is the evolutionary pressure leading to flying monkeys?
Is there is a new type of predator of monkeys?
In this case, the predator would probably wipe out the monkey population in its area. The reason being that if such a predator has higher than 60% rate of success for monkey hunting and reproduces quickly, the monkeys would just not get enough time to evolve flight in the first place. They would just go extinct.
And if the predator has a slow reproduction rate, a constant predator-prey relationship will form between that predator and monkeys. Deer have been preyed upon by tigers, cheetahs and leopards for tens of millions of years, yet they have no evolved flight. Their evolution is directed at faster speed and keener senses. Within monkeys, their evolutionary response to predation would also be targeted at increased speed, higher intelligence and keener senses.
The onus of justifying the origin of flying monkeys is upon you.
The monkeys could live in a landscape of groves that are too small to sustain a troop interlaced with treeless stretches. Monitor lizards could live on the open ground. They would be too heavy to climb up to the lightest branches, which would select for survival monkeys that are quite light. This would be aided by an absence of any aerial predators. The monitor lizards could be faster than monkeys on the ground. This would simultaneously force the monkeys to go to a grove and force them to attempt to jump to a grove. This would account for the ability of flight. The monkeys could later evolve migration after some climate event changes the weather pattern to a cycle of desert in the north and rains in the south for half the year and vice versa for the second half.