Mandrils were the servants of the Monking in former versions of the mod, although they are not present in the current beta, they are officially planned to return in future versions. However, in the new prerelease, you can use the monkey model and texture on an NPC to create something very similar to the original. In the 1.6.4 and older versions Mandrils could carry weapons and wear armor, similar to other Chocolate Quest mobs. This is still true on the NPCs.
Like real life mandrills, they had a blueish-reddish-yellowish snout. The sound they make sounds like monkeys screeching.