The Thinhorn Sheep is a medium-sized ungulate with a stocky body, slender legs, short ears and short tail. The body colour ranges from the all-white Dall’s Sheep to the dark blackish-brown forms of Stone’s Sheep, with a general cline of all-white individuals in the northwest to darker ones further south and east in the species’ range. Coat colour is the primary character used to distinguish the two subspecies.
Both sexes have horns, which are generally light brown or amber, with rings and ridges over their surface. Adult males have much larger horns than females, and they are roughly triangular in cross-section with an obvious keel on the upper, outer edge. Like those of most male wild sheep, the horn sheaths are sharply tapered and grow in a spiral out from the head so that in mature individuals they complete a full circle alongside the face. Most adult males – even the old ones – have a wide horn span and unbroomed (unbroken) horn tips. The female’s horns are short, gently curved up and back from the head, and elliptical in cross-section.