Okay so, basically the easiest way to tame these cute little guys are to keep your distance and make sure they don't face you. Put Mejoberries into the last slot of the hot bar and quickly run up every now and then, feed it, and then run back to a safe distance. It can be tricky but patience is key. Also, if you are a high enough level, dodo kibble can be used instead, if you wear a ghillie suit it has a lower chance of noticing you.
If you are going to tame these they eat berries (obviously cuz it’s a monkey)
but you need to crouch and every time you feed it you need to quickly turn around and walk away for a bit or the monkey will get scared and run away. Hope this helps