Hard keeper

[1] It is natural for a normal horse to require more nutrition to avoid weight loss if it is pregnant, lactating, under stress due to illness or management conditions, or when subjected to hard work.

[1][2] If an animal is truly a hard keeper, proper nutrition requires a calorie-rich diet, but one that will not make the horse "hot" and prone to excess energy that may lead to yet more nervous behavior and continued weight loss.

[3] High-energy feeds containing significant amounts of sugars, such as molasses, are not usually recommended because they have a tendency to make a horse "hot" or more excitable.

There are some body weight distribution changes that are linked to age, including a loss of muscle tone along the spine and hip that lead to somewhat more visible withers, hipbones, and ribs.

[6] In extreme old age, such as when a horse is over 30 years old, the animal may no longer have any molars left, and may require a diet of mushy foods such as hay cubes soaked in water, beet pulp, or other specialized feeds.

[2] In such cases, these horses will appear to be too thin, but if obtaining proper nutrition will still have a healthy hair coat, flesh over bone, and other indicators of good health.

However, the weight distribution and musculature of a hard keeper, particularly in the neck and hindquarters, is distinct from that of a starving horse, and a veterinarian can usually provide an expert opinion as to what is normal and what is not.

The Thoroughbred is a breed of horse that is naturally lean and can be difficult to keep in good flesh. This horse is in racing condition and is thin, but not too thin when considering the work that is being asked of it.
The mare in this image is too thin. She has increased nutritional needs because she is nursing a foal . She may be a hard keeper, or she may not be getting enough supplemental feed to support both herself and the foal.
A horse that is elderly and too thin even with adequate forage, such as this horse, may have an underlying dental problem or have trouble digesting a normal diet.
A horse that is not merely a hard keeper, but actually too thin