Similar to Bison wool, Yak wool is a very warm, lightweight and soft fiber. Yaks are primarily raised by nomadic Tibetan and Mongolian families. Their wool is combed (not shorn) once per year in the springtime when the animals shed their winter coat.
Khunu offer a range of athletic clothing that is made with 100% Himalayan yak wool. Khunu is based in Beijing, China, and run by Aaron Pattillo from the U.S., and Julian Wilson, a former British Army officer.
Khunu says that independent lab tests have shown yak wool to be 10-15% warmer than merino wool of Australia and New Zealand. They claim their wool has a luxurious hand feel which is comparable to cashmere, because only the ultrafine down fibers of the Yak’s coat are used (see more information here). Describing Yak down, the Fleece & Fiber Sourcebook says:
White yaks produce more fiber than darker ones, but because there are mostly dark yaks, white fiber is still far less common…[T]he fiber cannot be succinctly described, Some is crimpy, and some if more sleek, closer to alpaca than cashmere or qiviut (musk ox wool).
In sum, Yak clothing is expensive, but it is a good choice for for outings in very cold weather.