Yaks are shaggy-haired Oxen with short, stout legs. They are adapted to the cold and thin air.
Yaks are very valuable to the Tibetans: They provide meat, milk and butter. They are hardy beasts of burden, capable of long journeys in the high mountains in terrible weather.
This Monk is "walking his Yak"! From the looks of the Yak's Ceremonial clothes, he's going to a celebration.