The common treats that are given to horses are carrots and apples. Although, I have seen horses given chocolate, cookies, candy bars, lifesavers, hard candy, sugar cubes, soda and many other types of human sweet snacks.
Because the horse eats it, it is assumed that they like it and it can do no harm since the horse will not eat something that is not good for them. This is a myth. They eat it because it is sweet, for the same reason, people eat sweet snacks.
Feeding your horse sugar laden treats is more of a psychological comfort for people rather than doing something good for the horse.
Carrots and Apples, even though natural, have a high sugar content. Much of the pasture grass that horses graze on, originally was planted to fatten up cattle, and has a high sugar content.
All grains when digested are metabolized as sugar. Feed manufacturers add molasses to grains, pellets, cubes and supplements. This is a hidden form of sugar that should be avoided.
Feeding a horse grains or high sugar content human foods causes the pancreas to secrete insulin. Insulin is used by the cells to process sugar. Over time, the cell becomes over saturated with glucose resulting in a condition know as insulin resistance or IR because the cell can no longer process the high levels of insulin. If this continues the horse can develop Cushings.
Feeding a piece of candy or even several apples to your horse can trigger laminitis by causing hind gut acidosis which results in a metabolic shift in the micro-organisms of the digestive system. Inflammation of the laminae in the hoof occurs and your horse has sore feet or acts off.
Some horses may even have an allergic reaction to the sweet treats. They may develop hives, a sore back, sore feet, mood swings or stocking up.
The next time you want to treat your horse, instead of a carrot or apple, reach for a hay cube. The cubes can be a grass hay or alfalfa mix cube with no molasses hidden in it. Your horse will be happy and satisfied.