The mountain nyala (Amharic: የደጋ አጋዘን) (Tragelaphus buxtoni) or balbok, is a large antelope found in high altitude woodlands in a small part of central Ethiopia. It is a monotypic species (without any identified subspecies) first described by English naturalist Richard Lydekker in 1910. The males are typically 120–135 cm (47–53 in) tall while females stand 90–100 cm (35–39 in) at the shoulder. Males weigh 180–300 kg (400–660 lb) and females weigh 150–200 kg (330–440 lb). The coat is grey to brown, marked with two to five poorly defined white strips extending from the back to the underside, and a row of six to ten white spots. White markings are present on the face, throat and legs as well. Males have a short dark erect crest, about 10 cm (3.9 in) high, running along the middle of the back. Only males possess horns.

The mountain nyala are shy and elusive towards human beings. Four to five individuals may congregate for short intervals of time to form small herds. Males are not territorial. Primarily a browser, the mountain nyala may switch to grazing occasionally. Females start mating at two years of age, and males too become sexually mature by that time. Gestation lasts for eight to nine months, after which a single calf is born. The lifespan of a mountain nyala is around 15 to 20 years.

The typical habitat for the mountain nyala is composed of montane woodlands at an altitude of 3,000–3,400 m (9,800–11,200 ft). Human settlement and large livestock population have forced the animal to occupy heath forests at an altitude of above 3,400 m (11,200 ft). Mountain nyala are endemic to the Ethiopian highlands east of the Rift Valley, between 6°N and 10°N. Up to half of the total population of the mountain nyala occurs in the 200 km² (77 sq mi) area of Gaysay, in the northern part of the Bale Mountains National Park. The mountain nyala has been classified under the Endangered category of the International Union for Conservation of Nature and Natural Resources (IUCN). Their influence on Ethiopian culture is notable, with the mountain nyala being featured on the obverse of Ethiopian ten cents coins.

The mountain nyala was first described by English naturalist Richard Lydekker in 1910. Its scientific name is Tragelaphus buxtoni. The species has been classified under the genus Tragelaphus of the family Bovidae. The mountain nyala was first brought to England in 1908 by Major Ivor Buxton, after whom the species has been named. Major Buxton, who had returned from a hunting trip in Ethiopia, first presented a specimen he had shot to British taxidermist Rowland Ward, who later on notified Lydekker about the discovery. It was the last large antelope to be discovered in Africa. The spiral horns and white markings on the face and across the flanks established it as a species of Tragelaphus. Lydekker believed that the species was related more closely to the nyala (T. angasii) than to the two kudu species (then placed under the genus Strepsiceros). In fact, mountain nyala's similarity to both Tragelaphus and Strepsiceros led to the merger of the latter into the former genus.

In 2005, Sandi Willows-Munro of the University of KwaZulu-Natal (Durban) carried out a mitochondrial analysis of the nine Tragelaphus species. mtDNA and nDNA data were compared. The results showed that the mountain nyala is sister taxon to bushbuck (T. scriptus), sitatunga (T. spekii) and bongo (T. eurycerus) in the mitochondrial tree (phylogenetic relationships obtained using mitochondrial data). However, in the nuclear tree, bushbuck, mountain nyala and sitatunga plus the bongo are three equal branches. Hence the mountain nyala forms a monophyletic clade with these three species. The greater kudu clade split from the clade formed by mountain nyala, bongo, sitatunga and bushbuck approximately 8.6 million years ago.

The mountain nyala is a large sexually dimorphic bovid. The head-and-body length is approximately 240–260 cm (94–102 in) in males and 190–200 cm (75–79 in) in females. The males are typically 120–135 cm (47–53 in) tall while females stand 90–100 cm (35–39 in) at the shoulder. Males weigh 180–300 kg (400–660 lb) and females weigh 150–200 kg (330–440 lb). The bushy tail reaches the heel, and is 20–25 cm (7.9–9.8 in) long. The sensitive ears are large and lined with white hair.

The coat is grey to brown, marked with two to five poorly defined white strips extending from the back to the underside, and a row of six to ten white spots. White markings are present on the face, throat and legs as well. The white chevron between the eyes and the white patch on the throat are among the most conspicuous markings. The chest and the rump are white. The lower part of the legs are pale on the inside and clear white spots are present just above the hooves. The coat is fawn brown in male juveniles, and grows darker with age, eventually becoming charcoal in old bulls. Males have a short dark erect crest, about 10 cm (3.9 in) high, running along the middle of the back, from the neck to the tail. Coat texture may vary from smooth to rough, probably by season. Females resemble the red deer hind in size and proportions. Female juveniles are a bright rufous, and old females are as grey as young males. Females have two pairs of inguinal nipples.

Only males possess horns, whose maximum recorded length is 188 cm (74 in). Horns appear as cream-coloured nubs at around six months and start growing in a spiral pattern, reaching full growth by two years. The horns have only one or two spirals, though a few males have been reported to have two-and-a-half turns. The final form might vary in different males – the horns could form well-defined spirals, or could diverge in a structure similar to a lyre, resembling the horns of an impala, but with the final spiral incomplete. Growth rings are visible on the horn sheaths, but the annual patterns might be difficult to comprehend. Though the horns might wear out with age, the cream colour of the tips persists.