A city must have horses in its Strategic Resource box to build a mounted warrior.
Around the year 1600 AD, Spanish horses were introduced into the Americas and spread northward from the region of New Mexico, quickly covering almost the entire Plains and reaching the edges of the Eastern Woodlands by 1750. Horses revolutionized the hunting of game for the North American natives, making it much more profitable and allowing ever larger populations to move beyond subsistence. Although the Iroquois and other Eastern tribes did not use the horse in warfare as extensively as did the Plains Indians, nevertheless mounted warriors served as scouts and couriers during times of war. By the time of the American Revolution and subsequent destruction of the Confederacy, many of the chiefs of the Six Nations themselves rode into combat.