Keep electrical plugs and cords out of the reach of horses. Provide a vaccination program recommended by your veterinarian. A horse's digestive system is very sensitive and reacts to any sudden change in diet. Consult your veterinarian regarding the appropriate diet for a horse's age and activity level. Always provide shelter from bad weather with a clean place to lie down.
Horses really do enjoy lying down, despite the myth that they only sleep standing up.
If you don't have a barn, a three sided loafing shed can suffice as protection from the summer sun, flies, winter wind, and storms. This means scrub tubs and buckets regularly. Groom horse often to remove dirt and to stimulate the skin and coat. Horses are happier if they have company. If you own only one horse, perhaps a neighbor's horse can suffice.