One animal I have recently come to really want in the game is the
grivet monkey - unfortunately, it has zero votes on the meta-wishlist and has received scant attention at all.
Their cousins, the vervet monkeys, are slightly better-known but even then only have 12 votes on the meta-wishlist. According to Zootierliste, the grivet is the third most commonly-kept Old World monkey across Europe and Israel, while the true vervet is only kept in three zoos in Russia. In America, they are only slightly rarer than the vervet.
As well as being more commonly-kept overall than vervets, they also have a brighter coat shade. They have many of the behavioural features of vervets, being highly sociable, alert and intelligent monkeys that would be very interesting to watch in-game. They can also be mixed with in-game species - at one zoo in Germany they live with warthogs, lechwe and elephants, plus impala and bat-eared foxes. Their commonness means they would fit in well into a smaller zoo, and basically provide an African equivalent to the capuchin monkey - a mid-sized primate to serve as a more basic species compared to the much more impressive mandrill.
They would have fitted in perfectly to either the Africa Pack or the Grasslands Animal Pack. I guess they could fit in a Woodlands Pack, provided it includes tropical dry forests in it. A more unusual option, like a Primate, Urban or Horn of Africa Pack could also see the grivet being added.