I am not much of a primate lover (order primates, class Mammalia), but this particular species would be absolutely unique to have because instead of climbing it swings, the way it sits is very unique and it's overall behaviour is an animal that is curious about everything. They can also share habitats with the orangutans, and currently do in a few zoos. They sound really cool and have a large range of colours which gives a new animal with colour varieties to the game.
Benefits -
Unique climbing style (it's default mode of movement is swinging, it doesn't climb but instead uses a swinging momentum to get up high)
Unique sitting (arms flop to sides)
Runs on two feet on the ground, when it stops it squats
Comes in a range of colours - black, dark-brown,light-brown and sandy colors
New calling sounds
very playful and curious
Able to be housed with orangutans
Could bring new inter-species interaction animations