The statistics presented here can be used to represent any arboreal monkey that is no bigger than a housecat, such as a colobus, capuchin, spider or squirrel monkey.
Monkeys are highly social creatures. They spend the majority of their day searching for food.
5e SRD
Common monkey weighing about 30-40 pounds, usually shy but can be a nuisance
A Common Monkey weighs about 30-40 pounds. They are usually shy, but if one or more of them becomes comfortable around humans they can be quite a nuisance. This smaller type of monkey is generally inconsequential to adventurers.
Basic Fantasy Field Guide Omnibus