The Dryas monkey (Cercopithecus dryas), also known as Salonga monkey or ntolu, is a little-known species of guenon found only in the Congo Basin, restricted to the left bank of the Congo River. It is now established that the animals previously classified as Cercopithecus salongo (common name Zaire Diana monkey) were in fact Dryas monkeys. Some older sources treat the Dryas monkey as a subspecies of the Diana monkey and classify it as Cercopithecus diana dryas, but it is geographically isolated from any known Diana monkey population.
It is believed by the IUCN that only 200 individuals are left, although because the species is rarely spotted, an actual number is not known, leading this species to be listed as critically endangered. There are few speculations as to why this species has declined so rapidly and is not showing much progress when it comes to making a comeback. Some reasons include: poaching of the species for meat, habitat loss due to logging and other human activity, and the lack of information and knowledge of this species, makes them more susceptible to dangers.
The only conservation efforts known for Dryas monkeys comes from the community-managed Kokolopori reserve in the north-eastern Democratic Republic of the Congo. This reserve is key to the survival of this species.