Domestication shrank the yak considerably. A domestic yak typically weighs a few hundred kilograms, while a wild bull can approach or exceed a tonne and stand around two metres at the shoulder, with far heavier horns and a much more massive forequarter. Wild cows are notably smaller than wild bulls.
Both forms are built for altitude. Yaks have large lungs and hearts relative to body mass and blood chemistry suited to low oxygen, and they live and feed above four thousand metres year round. Their coats combine a dense insulating undercoat with long guard hair that hangs in a skirt to the ground, and they lack functional sweat glands, which makes them intolerant of warm lowland conditions.
The wild yak is listed as Vulnerable, with a population plausibly in the low tens of thousands. Hybridisation with free-ranging domestic yaks is a recognised threat, and whether wild and domestic yaks should carry separate species names is still argued.