Here are some general points:
* Most monkeys have some hair: They are all primates, and primates tend to be hairy.
* Hair serves many purposes: It helps with insulation, camouflage, and communication (like displaying a threat or attracting a mate).
* Hairiness can vary by species and even within a species: Some monkeys have very thick fur, while others have thinner fur. This can also depend on the climate they live in.
* Hair can change with age: Young monkeys might have different hair patterns than adults.
If you're interested in the hairiness of a specific monkey, you'll need to research that particular species.