They have to be shorter than 3 feet tall to be classified as mini. According to the American Miniature Horse Association (yes, this is a real thing), they cannot exceed a height of 34 inches at the withers (the end of the mane hairs). Mini horses can live up to one-third longer than average horses.
Are mini horses healthy?
Minis are very good at utilizing calories and most don’t need any sort of grain to provide appropriate caloric intake. Obesity can lead to increased incidences of insulin resistance, equine metabolic syndrome, and laminitis. Feeding hay at 1.5-2% body weight is ideal to keep your mini at an acceptable body weight.