L’Hoest’s monkey also known as the mountain monkey, is a guenon found in the upper eastern Congo basin. They mostly live in mountainous forest areas in small, female-dominated groups. They have a dark coat and can be distinguished by a characteristic white beard.

It was Philip Sclater who originally gave the name Cercopithecus lhoesti so as to name the species after of Michel L’Hoest, director of the Antwerp Zoo, at the time.

L’Hoest’s monkey is currently classified as a member of the genus Allochrocebus. Formerly, L’Hoest’s monkey included the preussi as it was classed as a subspecies from the Gulf of Guinea region, but it is now considered a separate species.

L’Hoest’s monkey was formerly included in the genus Cercopithecus, but molecular studies in 2003 questioned whether L’Hoest’s monkey was a member of the genus Cercopithecus. The studies indicated that L’Hoest’s monkey (along with the others in its species group) is more closely related to the vervet monkeys of the genus Chlorocebus and the patas monkey (genus Erythrocebus) than to the other guenons of the genus Cercopithecus.