Ideally a horse or pony should be kept with another equine friend.
A horse needs company when he is out in the field and needs to know
that there are others around when he is stabled. Many horse panic if left
on their own.
Sometimes a non equine companion is needed to keep with a horses when he is on box rest or is travelling.
If you cannot afford to keep another horse or pony, or don't have room there are a few options to consider.
If it is a question of cost - look at the option of advertising for a companion horse whose owner will pay to keep it with you.
Before you decide on a non equine companion for your horse or pony think about getting a miniature horse.
Miniature horses or Falabellas can make very good stable companions for a horse on box rest. They are happy to live in a sectioned off partition in a stable where they can be easily seen. If they kept separated but visible there is less danger of injury to either horse - but they are still aware of each other's company.
In a field a miniature horse can happily graze with his larger friend.
Some horse owners find that sheep or goats make good companions for horses. However soem horses can be very agressive towards sheep or goats and have been know to chase them out of their paddocks or kick them out ot their stables. Watch the film Seabiscuit to see the consequences of this!
Horses either love or hate goats - if they do get along with goats have the added advantage of eating the weeds that horses won't or shouldn't eat!
Some horses are happier grazing alongside cows than sheep or goats.
Often a few chickens, or a cat or dog around the stableyard can provide company.
Llamas or Alpacas have been tried as companions - some horses get on OK with them, but many horses and ponies are terrified of them! Definately try before you buy!