The aurochs, the ancestors of domestic cattle, is an extinct type of large wild cattle that existed in the real world until 1627.
It was known to have existed in the majority of Westeros but through hunting and domestication, the areas where it is now found in the wild are limited. One place that is reported to still have aurochs is near the juncture of the Kingsroad and the Green Fork of the Trident river in the Riverlands. They are frequently used as a symbol of size, strength, and stupidity. Grenn, a member of the Night's Watch, was given the nickname Aurochs, by Ser Alliser Thorne. An auroch was used as a mount by the mythical Clarence Crabb as a display of his prowess.