Personally I would be concerned about feral monkeys as much as I would standard every day raccoons. I've seen stories on TV where wild monkeys have caught and eatten birds and other small animals in the wild. They are definitely carnivorous/omnivorous and unlike raccoons, they are smart and can think. I would imagine that if a feral monkey could get it's hands and teeth (look at their incisors sometime. Those things are made for biting) on a plump juicey chicken, it would kill and eat it without hesitation.
Protecting our flocks from preditors is our responsibility no matter what we are protecting them from. And while my comment about getting your face bit off by one was meant as a joke, there are many instances where primates have inflicted painful and mutilating bites on humans.
Luckily where we live we are more concerned about the neighbor down the road's dogs getting loose and coming down to visit our dogs than we are anything else. Fencing is our best friend whether it be chain link or electric. There is no way we can free range our birds due to the number of red tailed hawks and owls that we have around us. We counted five in the air this week over our farm and that number will increase with the breeding season here. So our flock will be kept under wraps so to speak and since it's against the law to inflict punishment to the owners of the animals whether they be dogs, cats or monkeys, who turn the animials loose to torment folks who are just trying to get fresh eggs for their breakfast, the animal will unfortunately pay the price for the transgression if chickens wind up dead.