There is a reason that we measure modern-day engines in terms of horsepower rather than manpower or any other kind of power.
Horses were critical to the way people living in the "Old World" grew crops, moved around, and fought wars. The first Europeans to arrive in the Americas brought horses with them and those horses would become an integral part of farming here in the New World as well.
Before tractors, horses were the main source of draft power on many farms. They pulled plows and wagons. They hauled logs and stones out of fields. They transported the family to town. Horses even pulled the first combines. Many winches and pulley systems were designed to be powered by a horse walking in a circle.
Horses were, and are still today, used extensively on cattle ranches to move beef cattle. Horses became the first efficient rural postal service when the Pony Express was founded. Horses couldn't outrun a telegraph, though, so the Express was soon out of business. Horses, mules and donkeys were used as mounts and pack animals on the trails pioneers took west to start farms and ranches from St. Louis to the Pacific ocean. Horses provided a great deal of the draft power needed the lay the transcontinental railroads that now ship agricultural products from the heartland of America to the population centers on the coasts.
It's easy to look at our agriculture here in the US and think everything is mechanized, but estimates indicate there are still about 100 million horses, donkeys and mules used for agriculture and transportation around the world. Raising, showing, riding and racing horses is still a big business here in the US, with a devoted following.
December 13 is the "National Day of the Horse"--by congressional decree. You can read the full resolution
here, but the highlights of why Congress decreed a National Day of the Horse is because they are a "living link" to our history as a nation. The resolution continues on to say that horses are "a vital part of the collective experience" of Americans.
Whether it was Trigger, the Black Stallion, Zorro's horse Tornado, or a real-life equine hero like Barbaro, Secretariat, Alydar, or your own backyard pet--horses continue to inspire and captivate us as companions, competitors, and working animals.
Indiana's own James Whitcomb Riley said it well:
"I bless the hoss from hoof to head-
From head to hoof and tail to mane!-
I bless the hoss, as I have said,
From head to hoof, and back again!"