Mustangs
The image of horses running free across the plains is a popular symbol of the American West. However, mustangs are not native to the United States，they are descendants of Spanish horses. When Spanish armies came to the New World in the 16th century, they brought horses with them. Before the Spanish arrived，horses had been extinct in the New World for about 12,000 years.
The free horses that live in the western United States are called mustangs. The word comes from the Spanish word mesteno, meaning "wild". Mustangs are medium-sized horses. They measure around 56 inches to 60 inches(140 to 150 centimeters).They weigh around 800 pounds (360kg).
These horses escaped from the Spanish soldiers. And they were also used by native people to fight against the Spanish army. As time went on, the horses gave birth to baby horses and were joined by other horses.
By the end of the 19th century, about two million mustangs ran across the countryside. Farmers began to complain. The mustangs were destroying their crops. The mustang population decreased as farmers drove the horses away from the plains. Then, private conservation efforts to protect the mustangs began as early as 1925.
By 1970, fewer than 17,000 mustangs were left. As a result, the United States passed a law that protected the horses. Now, it is estimated that more than 35,000 mustangs are living in the United States today.