The mustang is a free-roaming horse of the American west that first descended from horses brought to the Americas by the Spanish. Mustangs are often referred to as wild horses, but because they are descended from once-domesticated horses, they are properly defined as feral horses. The original mustangs were Colonial Spanish horses, but many other breeds and types of horses contributed to the modern mustang, resulting in varying phenotypes. Free-roaming mustang herds vary in the degree to which they can be traced to original Iberian horses. Some contain a mixture of breeds, usually from ranch stock released for various reasons, others are relatively unchanged from the original Iberian stock.