The American Mustang can be any type of wild horse. The term Mustang comes from the Spanish word "mesteno," which means "stray or ownerless" horse. An American Mustang is any wild horse in the United States. Today, there are about 41,000 Mustangs that wander freely on public land after Congress passed the Wild Free-Roaming Horse and Burro Act in 1971. Mustangs are described as "living symbols of the historic and pioneer spirit of the West."