Being overweight or underweight can pose health problems for a horse, although a slightly underweight horse is probably a little better off than an overweight horse. However, it takes very little — illness, overwork, stress — for the slightly underweight individual to become too thin.
Many plants and flowers are poisonous to horses, including rhododendron, milkweed, foxglove, laurel, yew, nightshade, bracken fern, ragwort, buttercup, lily of the valley, narcissus, and larkspur. If you see a sudden onset of diarrhea, colic, extensive salivation, staggering, or muscle weakness, or if the horse collapses, suspect poisoning and call your vet immediately.
Overweight individuals tire easily, and obese broodmares can have breeding difficulties. Some horses, like people, easily gain weight and need to be carefully monitored for food intake. They may need to be taken off pasture, placed on restricted turnout, or fitted with a grazing muzzle and given a more controlled feeding program. They will probably be more inert, so the owner will have to take responsibility for getting the individual the exercise it needs to maintain a good weight. Keep close tabs on your horse's weight, and work with your health care team to address any weight issues.