The Old World monkey subfamily that is the only primate group with foregut fermentation is Colobinae.
Colobines include langurs, leaf monkeys, snub-nosed monkeys, and proboscis monkeys. Their complex, multichambered stomachs house microbes that ferment plant material before it reaches the intestines. This adaptation helps them extract nutrients from fibrous leaves and detoxify some plant compounds.
Their digestive system is one reason colobines can specialize in a foliage-heavy diet. They are not exclusively leaf-eaters: many species also consume fruit, flowers, seeds, or occasional insects. Foregut fermentation distinguishes them from the other major Old World monkey subfamily, Cercopithecinae, which includes macaques, baboons, and guenons.
A common mix-up is with howler monkeys. Howlers are New World monkeys and also digest leaves efficiently, but they rely on hindgut fermentation in the colon or cecum rather than the colobine-style foregut system. Callitrichidae is a New World monkey family, while Homininae contains humans and the great apes.