Horses are by nature herd animals. However, the one horse you need to keep separate from the herd are stallions because they are driven by testosterone and can become unruly. They are not recommended for first -time horse owners. Many boarding barns will not even allow you to board if you have a stallion. If you get a youngster, you ask the vet to geld him (which means to castrate him).

Mares (I have heard) tend to be a bit bossy. I  have geldings and they get along together just fine.

If you have to only have one horse, with no intention of getting another, you might want to consider a companion animal like a goat, or a miniature horse to keep your one horse company. They do find comfort in a companion of some sort, and you should try and spend as much time as possible with your horse as well. They do tend to get bored easily.

 Horses have the type of nature, they either are flight or fight animals. Mostly they flee. They are scared of everything, thinking it is a huge predator whose main purpose in life is to eat them! In a herd situation, this type of behavior is evident. There is always a lead horse, looking out for the rest, there are submissive personalities, and those who bully if they can get away with it. Watching two horses rearing and playing, biting and kicking can be a bit daunting the first time around, but usually it is just extra energy and playfulness and not an all out aggressive attack.

No matter how your horse responds in a herd situation, you must be the stronger of the two personalities. You move into that role, not by cruelty and physical strength, but by understanding your horse and letting him know in various ways that you are the Alpha. You earn his respect, because once you lose it, you may never be able to get it back. Horses have long memories. People should be so fortunate!