The horses of the Northern Plains Indians were different than those of the Souther Plains Indians. These were heavier boned and rangier than the Southwestern variety.
The reason is the influence of the Canadian Horse, known as "The Little Iron Horse." This critter could withstand harsh weather, scarce food and was a tough, all around working horse.
The Plains Indians bred this Canadian horse to their little Spanish horses and it increased their endurance. Sitting Bull was a breeder of these horses, and today, these horses' bloodline still exists in the rolling hills of North Dakota.
The Plains Indians have a spiritual bond with their horses. And some believe that horse have always existed in North America.