Think of horses like dogs with a vast array of breeds and temperaments.

There are breeds and varieties of horse that are more or less sensitive/excitable. That’s why we refer to some as “hot bloods”, “warm bloods” and “cool bloods”. Race horses like arabians or thoroughbreds fall in the hot blooded category. They’re sensitive and excitable—that’s why they’re good at running fast on command—it’s prey/flight behavior. Draft horses like clydesdales fall in to the cool blood category. They’re very big and calm and stable and good for work. Warmbloods are prolific in dressage, which is based on military riding disciplines. You want them a little fast and flighty, but also a little calm and collected. They can often be big, too, because they were bred to carry knights and artillery into battle (edit: i may actually be thinking of cool bloods that were bred to make modern warmbloods, but i digress).

War horses are going to come from less temperamental hot-blooded breeds. From there, you also have a very very social animal that can be bonded to a rider and trained for certain jobs. Horses are herd animals and adept at working as a unit/following a leader. Instinctually, some horses know that sticking out from the crowd is deadly.

Horse breeding and training in general were already ancient arts by the time of World War I and thus included breeding and training horses specifically for war.

As far back as the Middle Ages, there were already distinctions between warhorses, riding horses, and labor horses and there were also different kinds of warhorses. There were destriers (large, heavy, very expensive warhorses meant for knights), coursers (lighter and less expensive warhorses meant for knights and men-at-arms), rounceys (general purpose horses meant for riding and sometimes also for war), palfreys (riding horses only, often used by women), and cart horses (working horses used mainly for plowing and agricultural labor).

Even today breeders and horse trainers select animals who seems undaunted by sounds, and then they expose them to more to train them. They still do this to train police horses.

Before the war, they were considered necessary in an army. But from the development of artillery and machine guns, it was quickly realized that they maybe shouldn't, and were replaced by tanks at the same time. Horses during WWI were used mainly for reconnaissance and sending messengers, as well as pulling artillery, ambulances and supply wagons. The benefit of them was that they could travel better over muddy or rough terrain. WWI was kind of the turning point at which they stopped being in use, though they were used a bit in the beginning.

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