Horses are herd animals.
The horse needs the companionship of other horses so they do not become mentally depressed or unbalanced. They are a social animal that must have a herd member as a friend. A horse can often develop a herd type relationship with another horse even if they are separated by a fence, wall, road or other obstacle. That being said, how do you become a horse owner if you can only afford one horse and you opt to keep that horse at home? The answer might be simpler than you thought.
Other animals that gather together in herds can be substituted as a second horse. It is very common for racehorses to have a goat for a companion. Other animals that can be kept with a horse for companionship are miniature horses, donkeys, burros, llamas and sheep.
I found a humorous youtube video that a burro sings about the animals she babysits, or keeps company with in the barn. I have it posted below.
Don’t forget, horses will act differently to the introduction of a non-horse buddy in comparison to another horse, so you must be careful during the introduction stage.