[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.