Horses are social herd animals. In a perfect world your horse would have a lifelong group of equine buddies and never have to be by himself. In the real world, horses are sometimes alone. A horse may live alone or may travel off by himself to events and competitions.
Since horses are herd animals, most of them thrive better with a buddy that helps keep them socially engaged in activities throughout their day when you are not with them. New window. 
Some horses handle that solitary life and travel just fine while others pine away or develop stable vices. The solution is often to get a companion for your horse.
You can read about some exotic equine companions such as Jojo the monkey who accompanied the famed Seabiscuit on his travels from racetrack to racetrack.
There are more common and reasonable choices however. The top five equine companions are dogs, cats, goats and other small ruminants, poultry and small equines.
A dog as your horse's companion
Dogs are great horse companions as long as your dog understands he is not to chase or nip at your horse. A well behaved dog can accompany you on trail rides or ride in the carriage with you on drives.
Golden retrievers and other friendly, well-behaved dogs make excellent companions for some horses.
© 2015 by Thomas Lieser New window. 
A dog will generally stick around his home barn and some are even good about travel and staying at the tack stall or trailer at shows.
Also, a dog adds a level of security - barking if a stranger approaches your horse or your area. Many horses develop close bonds with the barn mascot dogs.
The drawback is that dogs often prefer to be in the house with you more than being out in the barn with your horse!
Still, many horses are happy just to have the dog around at the barn for extra company when you are away.
However, if you have a horse who gets worked up by a barking, leaping dog, that same horse may be calmed by the quiet of a cat.
Cats as buddies
If you want a great dual-purpose pet, think cat. A great barn cat will keep rodents at bay, saving your grain, hay and even tack from the insatiable appetites and chewing of rodents.
On cold nights you might find your barn cat curled up next to your horse in the straw.
Many cats become quite close to their equine friends and vice versa. There are numerous adorable photos of a cat rubbing up to a horse while balancing on a fence post or winding between the legs of a steady horse while on the cross ties.
Cats do not tend to make good travel companions however. Cats get attached to places as much as to other living beings and can't be counted on to hang at the trailer or around the stalls at a show site. They prefer to hold down the fort at the home barn.
Other delightful pals
Small ruminants, with goats leading the list, are next in popularity as horse pals. Sheep and cows, especially small breed cows such as Dexters, can be excellent horse buddies. They can share a pasture so they graze contentedly side by side all day. You don't have to buy a lot of separate food or even arrange for a different veterinarian in many cases.
As fellow herbivores, they share some of the same behavioral characteristics with horses and also want a "herd".
Goats lead the list as small ruminant pals wince they share some of the same behavioral characteristics with horses and enjoy being part of a herd. New window. 
Goats are popular because they are clever and cute. Adding a Pygmy goat seems like an easy solution until you have to deal with the caprine tendency to be escape artists. A small goat can escape through even more places than a full size goat!
Goats also tend to get into mischief. While the old stories of goats eating tin cans aren't true, goats do eat a lot of unusual items.
Poultry can also be multi purpose horse companions. Chickens are known for their insatiable appetite for ticks. Guinea fowl are also big tick eaters and can serve as sentry/alarm animals as well.
Geese are known for their bug eating but they are also the poultry equivalent of a Rottweiler when it comes to watch dog tendencies. Poultry may also leave you some wonderful eggs for your breakfast.
Drawbacks to poultry include their dander, which causes allergic reactions in some horses. They will require special feed and may need their own stall. They can be messy.
While poultry do provide company for your horse, most poultry do not get attached to a special horse. they are simply part of the farm "neighborhood". Still, many horses simply enjoy watching chickens, ducks and geese and like having them around.
Small equines as chums for a lonely horse
Last but not least are the "small equines." Miniature horses and miniature donkeys can be excellent companions for a lonely horse. They will eat the same foods and can stay in the same pastures. Depending on your barn set up they may share a stall or happily reside in a smaller space next to your horse's stall.
Some horses are put off by donkeys and don't bond well. Whether that is due to braying is not known. Donkeys can serve as "guard animals" in some respects.
A normal size donkey may help to keep stray dogs and wild canines off your property. Miniature horses and miniature donkeys should not be counted on as predator deterrents.
Above all, consider your horse!
Choosing a companion will depend partly on your horse. After all, if the newcomer is to be his buddy, he should have a say. You may have to try a couple of different animals to find just the right fit.
Did you know?
Goats can travel with you and may be content to stay in the stall or the trailer while your horse participates in shows.
Providing a companion for your horse will mean some extra costs for feed and veterinary care plus possibly some extra fencing.
You might get additional benefits such as fresh eggs or added security but you need to consider this an expense for your horse's mental well-being. It is less expensive than adding another horse and the psychological benefit to your horse is priceless!
About the author
Deb M. Eldredge, DVM is a Cornell graduate and horse lover from early childhood. She was active in 4-H and Pony Club, riding mostly huntseat but also Western. She has competed in various horse show venues as well as competitive trail rides and small three day events. At Cornell she was a member of the Women's Polo team.
Dr. Eldredge is a national award winning writer from both the Cat Writers Association and the Dog Writers Association of America. She lives in upstate NY on a small farm with 3 elderly horses, 1 miniature horse and 2 donkeys as well as various other animals.
Visit Deb Eldredge's Google+ Page