They can ride in your shoulder. Having a monkey while sailing in your pirate ship is strangely satisfying. 😎

If you put them on wandering they'll pick up berries. When you put it on your shoulder it throws poop at your enemy and the poop does damage and slows down the target

If you put stuff in its inventory like eggs or materials and then put it on your shoulder it will weigh less than it would with the items in your own inventory making it perfect to cook kibble fast

Despite what others have said, they do NOT throw arrows.

How to rule the sea:
Step 1. Tame Monkey
Step 2: Build boat
Step 3: Put Cloth Hat on said Monkey
Step 4: Sail the seas with your pirate monkey.
Congrats u r now pirate.