If you are wondering if you should keep the colt you bought, or one your mare just produced a stallion, the answer is likely 'no'. There is rarely justification to keep a backyard horse or a home bred colt a stallion.
Many horse owners feel there is romance, prestige and glamour in keeping and riding stallion. However, the reality is that keeping a stallion is a large responsibility that carries several risks. Stallions need not be dangerous if properly handled, but poorly handled ones often are. If you really want to own a stallion, first of all, there are safety considerations.
Your colt may be sweet tempered now, but once his hormones kick in, his behavior towards you and other horses may change. Handling a stallion is not the same as handling a saucy colt. While you may be able to reprimand your colt when it misbehaves, a stallion may not respond in the same way, and unless you are an experienced handler who knows how to maintain a stallion's respect, you may find yourself on the losing ends of the negotiations.
Young stallions can be more difficult to train. It's important to remember that a stallion's first priority is to breed mares. It can be difficult to keep a stallion's mind on his lessons when he suspects there is a mare in heat back in the barn. Training can take longer, and the lessons may not 'stick' like they would for a gelding or mare.
There are some things you may not be able to do with a stallion. Riding him in a group, such as trail riding, in a parade or at a show may be risky as he will consider all mares his, especially those in heat. If he decides that he must breed a mare, he may attempt to do so regardless of whether the mare has a rider or not. He may be aggressive towards geldings. If you encounter another stallion (at a show perhaps) you will need to be very cautious. All of these situations require the handler or rider of a stallion to be constantly attuned to the horse, see potential problems before they arise and a have knowledge of how to deal with any situation.
Breeding can be risky. If your stallion injures a mare while hand breeding or pasture breeding, the expense of treating the injury may be yours. Very serious injuries, such as broken legs have happened during breeding. And it's not just the mares (or your stallion) that can be injured. Handlers are also at risk.
Usually, stallions must be kept separated from other horses. Yes, there is an exception to every rule, but there is no guarantee your horse will be the exception. Some live amicably with mares and geldings. However, this is rare. Stallions shouldn't be stalled next to mares in heat (unless they are teasing). Because they could be aggressive towards geldings, they should not be able to reach them over fences or stall walls. If you board your horse, you may have difficulty find stabling at all. Most boarding stable owners understand the effort and risk associated with keeping a stallion and don't want the trouble.
Beyond the safety concerns, you must also be responsible for your stallion's reproductive health and the health of the mares he breeds. This means regular veterinary care and vaccinations beyond those normally given to horses. You need to know that your stallion is free of sexually transmitted diseases such as Equine Viral Arteritis, and how to keep him that way.
Keeping a stallion also means you have a responsibility towards his offspring. There is no shortage of horses in North America. Currently, there is an outcry about sending horses to slaughter and horse rescues are filled to capacity. If anything, there is an over-population. A stallion is capable of breeding around two hundred mares every year. That means your stallion could possibly contribute up to two hundred new horses into the population every year. Does the world need two hundred more horses?
Part of your responsibility towards your stallion's offspring is his ability to produce foals that have good conformation, temperament and bloodlines. Your stallion should also have a proven performance record that confirms that his genetics are worth reproducing. That he has successful ancestors is not sufficient. Showing and promoting your stallion takes time and money. He needs to be held up against his peers—horses of the same breed or type, and prove that his qualities are worth passing on and are marketable. To breed horses of poor quality is a form of cruelty. Consider what happens to poorly conformed, bad acting horses of no particular breeding or performance record.
But, you say, I love my colt, and I want more just like him. The likelihood of your producing a 'clone ' of your horse is highly unlikely. In a family of ten children, how many are exactly the same as their parents? In the families I know of, large and small, there are members who are somewhat similar, or not at all. You're also unlikely to produce a carbon copy of any particular horse.
If you feel you must own a stallion, try working at a breeding barn for a while. You'll see how the mares and stallions are handled and what it takes to maintain a breeding stallion, including housing, feeding and handling one. You'll see how carefully potential stallions are selected for their outstanding temperament, conformation, performance records and demand for their bloodlines. Any foal produced must be produced with an eye towards marketability. Once you understand how to select colts and handle stallions and mares for breeding, you can begin searching for a colt that meets your own strict criteria for improving the horse population.