Pony Rider - William Jackson - National Park Service
The Pony Express was formed because of the Westward Expansion of the United States. The discovery of Gold in California and general westward migration created a need for a fast and reliable mail service. Outfits like the Butterfield Overland Mail Service and private carriers partially met the need. But it was not until 1860 that an efficient system was put in place. This was known as the Pony Express, and it originated from the creation of the Leavenworth & Pike's Peak Express Company (National Park Service)
Hollywood gives a good view of the Pony Express in an early movie called the Winds of the Wasteland. It is worth a watch, just click the link.