For any number of reasons horses can begin to gain weight. Often horses that have been hard working or regularly exercised horses that suddenly find themselves turned on pasture, retired or even recovering from an injury will begin to put on weight due to the body's inability to change the metabolism to account for the decrease in calories burned.
Ponies are often very prone to weight gain due to being bred to have slower metabolisms. Many of the pony breeds excelled and thrived in areas where horses would have starved and over generations have adjusted to be able to live on very small amounts of low quality feeds. In modern, domestic environments with lush pastures and daily grain, these ponies simply pack on the pounds, even with regular exercise. Some horses and ponies are also given a huge number of treats, supplements and feed additives that are simply not needed or even healthy for the horses. Sugary treats, treats high in carbohydrates and fats can all lead to adding weight to your horse if they are fed in moderate to excessive amounts.
Horses have also been bred to put on muscle mass. Many of the heavy horse breeds or draft breeds have been selected just for their ability to put on weight, which, if the horse is not used, will be largely fat storage. Horses of all sizes are born to live completely off grass, provided all the protein, vitamins, energy and minerals needed are present in the pasture. Adding grain, alternative feeds, pellets and even supplements is perfectly fine, as long as it is done in balance and in reasonable amounts.
Working horses will, by definition and common sense, need more feed and higher quality feeds than horses that are in the pasture most of the time and only used occasionally for pleasure riding. While there is no hard and fast rule about how much exercise is necessary for a horse, most breeders and vets recommend free access to a large paddock or pasture, the companionship of other horses as well as regular exercise of at least 30 minutes per day for horses that are used occasionally.
Getting a horse to lose unwanted weight is a combination of decreasing caloric intake and increasing exercise. Since obese or overweight horses have more problems with joint and muscle injury gradually increasing the work time and watching the horse for signs of stress, discomfort or fatigue is essential when increasing exercise. Avoid exercising in the heat of the day or on hard surfaces that could lead to leg and hoof problems.
Overweight horses should be provided lots of fresh, clean water and access to good quality grass pasture or grass-type hay. Grain and pellet feeds should be minimal in the diet, ideally not present at all until the weight is at the desired level. Instead of sugary treats or commercial horse treats, try providing some sliced carrots, apples, broccoli or celery as a treat or reward. Each horse will have his or her own favorite fruit or vegetable, so do a bit of experimenting to find out what healthy snacks you can offer your horse.