This is a hugely controversial issue that I would like to weigh in on. A horse should not be kept as a pet in a pasture. Equine animals are work animals and enjoy having jobs, goals, and a reward. I have heard of many cases of people just keeping horses in a field and I personally think it's cruel.
Horses often become out of shape and lose important muscle mass. Not only that, but horses are thinkers and they get bored easy! They need something to do just like people. If they are just left in a pasture or stall they may exhibit bad habits like digging, cribbing, weaving, and pacing.
A horse is not a machine and should be treated humanely. They are not ONLY there to work. A working animal is not only for our benefit, many believe that a horse would be better off if they did not work at all. However, it is about what is best for both horse and human.
I benefit mentally and physically from riding and the horses I ride get a good romp. A one to two-hour ride every day or every other day will not compromise the welfare of your animal. In fact, it actually improves it. I am sure humans would enjoy being a couch potato as a career but, like humans, horses have a purpose.
Please don’t misunderstand what I am saying. A horse loves being a horse. They love to romp, relax, and browse the field. However, horses like work too. Many animals aim to please their human. Horses and humans share a symbiotic relationship that has stood the test of time.
There are a few exceptions to this rule. If a horse has done his fair share of work in life, then it is acceptable and encouraged to retire him. This also goes for animals with ailments that put them out of work. They should be given a comfortable, easy life. If it is a happy, healthy 5-year-old quarter horse mare, then there is absolutely no reason the animal should be left untouched. These graceful beings should be allowed to partake in disciplines and allowed to reach their full potential.