The original mustangs . . .(a free-roaming feral horse) were Spanish horses or of Spanish-descent, and over the years became a mix of numerous breeds. As European settlers moved farther west, they brought their horses with them. Some horses were lost to Indian raids, while other horses were freed as wild stallions tore down fences to add the tame mares to their herd. Tame horses escaped from early settlers just as the original horses had escaped from the Spanish. Native Americans also bartered and captured horses between tribes, making the distribution more complete.

Herds of wild horses from the Eastern United States were forced west by civilization and eventually crossed the Mississippi River and joined the western herds.

When reading about the history of the North Dakota Badlands, it is noted that thousands of horses were driven in herds to this area by cowboys such as "Badlands Bill" McCarty from the Southwest United States. "Badlands Bill" would take a crew of cowboys down the trail and return with a herd of 1,000 to 1,500 horses that were kept in this area until they were trained and sold.

Some of the small herd of horses (approx. 250) confiscated from Chief Sitting Bull and later sold were turned into the area known as the North Dakota Badlands. There is little if any history to verify if any of these bloodlines exist in the horses that were within the boundaries of Theodore Roosevelt National Park when the park boundaries were fenced.

The horse herds within Theodore Roosevelt National Park have been observed for more than 60 years. Over the years the National Park Service introduced a few horses within the park, with very little success. The park horses, as we know it for the past 30 years, is a breed made up of many horses that have been able to survive and flourish in the extreme harshness of the North Dakota Badlands.