The Pronghorn of North America, though sometimes known colloquially as Pronghorn Antelope, is not a member of the family Bovidae, but the family Antilocapridae and not a true antelope. No antelope species are native to the Americas. True antelope have horns which are unbranched and never shed, while Pronghorns have branching horns, and shed annually. Each "horn" of the Pronghorn is composed of a slender, laterally flattened blade of bone that grows from the frontal bones of the skull, forming a permanent core. As in the Giraffidae, skin covers the bony cores, but in the Pronghorn it develops into a keratinous sheath which is shed and regrown on an annual basis.
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Unlike the horns of the family Bovidae, the horn sheaths of the Pronghorn are branched, each sheath possessing a forward-pointing tine (hence the name Pronghorn). The horns of males are well developed. Males have a prominent pair of horns on the top of the head, which are made up of an outer sheath of hairlike substance that grows around a bony core; the outer sheath is shed annually. Males have a horn sheath about 12.5–43 cm (mean 25 cm) long with a prong. Females have smaller horns, ranging from 2.5–15 cm (average 12 cm), and sometimes barely visible; they are straight and very rarely pronged. It can run exceptionally fast, being built for maximum predator evasion through running, and is generally accepted to be the fastest land mammal in the Western Hemisphere. The top speed is very hard to measure accurately and varies between individuals; it is variously cited as up to 70 km/h, 72 km/h, or 86 km/h. It is often cited as the second-fastest land animal, second only to the cheetah. It can, however, sustain high speeds longer than cheetahs. - courtesy of WikiPedia