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Man began using animal power beginning with oxen yoked in Mesopotamia and Egypt. Horses later were used for transportation but still the ox did the plowing, mill work etc.
Egypt 1920 BC From a tomb at Beni HasanOx Yoke
In the Medieval building accounts of Troyes in the 13th century AD, in an average trip, a pair of horses pulled wagons weighing 11,000 pounds (5000 kilograms) with stone from quarries 31 miles (50 kilometers) away.
This is a very heavy load in comparison with the maximum load, 1100 pounds (500 kilograms), a pair of horses were authorized to pull on roads in Roman times.
The Roman Theodosian Code of 438AD decreed that anyone caught harnessing horses to a load in excess of 500 kilograms would be severely punished. So exceptionally low was Roman horsepower that horses were never used in agriculture. How did the French in Troyes 800 years later during the dark ages use horses to pull so much?

It's in the harness or collar.
Horse Collar

In about the fourth century BC the Chinese made a great breakthrough. A painting shows a yoke across the horse's chest, from which traces connect it to the chariot shafts. Soon, the hard -yoke across the breast was also abandoned and replaced by the more satisfactory breast strap, commonly called the 'trace harness'. There is no longer a strap across the horse's throat; the weight of the load is borne by the horse's chest and collar bones.

Lefebvre des Noette's book "L'Attelage—le cheval ä travers les altes" in 1931 he claimed that the Greeks and Romans had never found the correct way to harness horses but had simply adapted the yoke harness of the ox, with minor modifications, to the horse without ever realizing how inefficient this system was. As soon as the horses started to pull, the neck straps pressed on their jugular veins and windpipes, strangling them and making them throw back their heads like the horses of the Parthenon. Lovers of Greek art have always celebrated the rare genius of the Greek sculptor who gave such dignity to the horse, without realizing that the horse they celebrated had his head raised to avoid strangulation.
Measure Horse Collar
In 1910, Lefebvre des Noettes conducted a series of experiments in Paris to verify his theories. He harnessed horses as they are depicted in Greek and Roman monuments and found that a pair of horses harnessed in this fashion had difficulty in pulling loads in excess of 500 kilograms, proving the merits of the regulations in the Theodosian Code.
The correct way of harnessing horses was to build rigid padded collars, which would rest on the shoulder blades of the animal and not interfere with its breathing. This "modern" harness seems to have been used for the first time somewhere in the steppes separating China from the Siberian forests, and was originally designed for camels. It was introduced into Europe sometime in the eighth century, and in 800 we find its first European application. By the end of the ninth century we hear of horses used for plowing on the northern coast of Norway, no doubt with the new harness. This is the very first mention of horses employed in agriculture. The earliest representations of horses working in the fields appear in the border of the Bayeux tapestry (eleventh century ), where there is one pulling a harrow; and a tapestry of the Creation in the Cathedral of Gerona from around the same time shows a team of horses plowing the Land with a revolutionary heavy-wheeled plow. From then onward an increasing number of works represent horses plowing with the modern harness. And the horses no longer raise their heads.
Other studies concluded that The Dorsal Yoke, The Tandem Hitch and The Neck Yoke all of Roman use worked as well as the later horse collar, with the possible exception of plowing.

sources: Wikipedia, The Medieval Machine by Jean Gimpel
http://www.humanist.de/rome/rts/ by Dr. Judith A. Weller

Spruytte, pp. 13-15 and extra drawings by Dr. Judith A. Weller