From the Wikipedia Article, here is some history of the breed.
The Clydesdale is thought to have arisen from the mid-18th century cross breeding of local mares with larger English and Flemish stock perhaps originally developed for use as warhorses. They were further developed to meet the practical needs of early 19th century Scotland, where the robust constitution, somewhat shaggy coat, thick mane and feathered legs were suited to the climate.
The breed was well received owing to its agile strength and docility, soon spreading to northern England where it was used extensively to tow coal skips. In the 19th century Clydesdales were exported to Australia, and New Zealand. In Australia, with British Longhorn Cattle, they were an important draft animal. The Longhorn is forgotten in Australia, but the Clydesdales have survived a period of mid-twentieth century neglect to become regarded with the Merino as an icon of Australian rural industry. The Clydesdale is celebrated in one of the most popular images of rural life, G.W.Lambert's painting "Across the Black Soil Plains" which shows a team straining to pull a wagon loaded with the wool clip which is "up to its axle-trees" in mud. Nowadays they are one of the most popular exhibits at the agricultural shows and the Carlton and United Brewery Clydesdales, which are stabled at the Sydney Showground are visited by many thousands of people in conjunction with the Royal Easter Show each year. In New Zealand, apart from general rural work, the Clydesdale was used extensively in the timber industry, to pull from the forest the valuable logs of kauri pine, highly prized for cabinet making.
Clydesdales were first shipped to North America in 1840, and later to South America, Russia, Austria and Italy. Exports peaked in 1911 with a recorded 1,617 stallions trading hands. According to the Clydesdale Horse Society (formally founded 1887), between 1884 and 1945 20,183 animals were exported. Two stallions are recognized as the foundation of the breed: Lord Darney and Prince of Wales. All Clydesdales horses today can be traced back to these two sires. The development of the breed has come a long way from these two foundation sires. There was a lot of focus on developing the hind leg and quality of hair.
As a beast of labor, Clydesdales had been largely replaced by tractors and other heavy machinery by the end of World War II. Through the determination of many small breeders the breed continued through the lean post World War II era. However, the horses are still used in situations where machines are unwanted or inferior, such as "eco-friendly" farming and logging operations. Clydesdales are now most often seen in competitive agricultural exhibitions such as state, county and national fairs.