Horses evolved to travel large distances and to graze for much of the day. Standing still in an enclosed space goes against the grain. Being confined to a stable will soon lead to frustration. Horses may suffer from emotional distress if they are not sufficiently stimulated and can become dangerous to handle. If your horse is stabled, it is important to do everything that you can to support their need for stimulation and exercise.
Exercise
Evolve a suitable exercise plan unless illness or injury necessitates stable rest. . Plan ahead to ensure that you can ride out in daylight or have access to an arena. If you do not have access to a paddock, think about whether you could turn your horse out into an arena or an area of the yard. If you do this, provide a supply of hay and arrange for your horse to spend time with a friendly horse so he has company. You may be able to create a fenced area outside your horse's stable. Your horse will then have the choice to wander in and out when he pleases and will feel less restricted. You should also take your horse for walks but be prepared for him to be full of beans. Wear your riding hat and gloves and let someone know what you are doing and where you are going. You should ideally attach a lunge to his headcollar and take someone else and their horse along with you.
Environment
A change can be as good as a rest. If there is a spare box or stall at the yard, could you swap for a while to give your horse a change of scenery? If possible, tie him up in different places around the yard to give him variety but only use safe areas. Horses are sociable animals and so it is always good to stable your horse next to a friendly horse so they can interact. They can see, touch or groom each other throughout the day and won't feel so isolated. If your horse already has a friend but they are not stabled nearby, take your horse to visit them whenever you can so they can spend some time together.
Entertainment
When there is no option but to leave your horse in his stable, provide some toys to keep him occupied. You can purchase a variety of toys, stable licks and distractions that stave off boredom. Change the toys regularly to keep your horse interested. It is vital for your horse's health and emotional well-being that they receive the exercise and stimulation that they need. This may require careful planning and a great deal of effort. But it will be well worth it when you have a contented and healthy horse.