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    An estimated 400 horses in total were used by the Pony Express to deliver the mail. Horses were selected for swiftness and endurance. On the east end of Pony Express route the horses were usually selected from U.S. Cavalry units. At the west end of the pony Express route in California, W.W. Finney purchased 100 head of short coupled stock called "California Horses"' while A.B. Miller purchased another 200 native ponies in and around the Great Salt Lake Valley. The horses were ridden quickly between stations, an average distance of 15 miles, and then were relieved and a fresh horse would be exchanged for the one that just arrived from its strenuous run.

During his route of 80 to 100 miles, a Pony Express rider would change horses 8 to 10 times. The horses were ridden at a fast trot, canter or gallop, around 10 to 15 miles per hour and at times they were driven to full gallop at speeds up to 25 miles per hour. Horses of the Pony Express were purchased in Missouri, Iowa, California, and some western U.S. territories.

http://en.wikipedia.org/wiki/Pony_Express