A horse is a horse, of course, of course...
Horses are lively and high-spirited. They are happy in fields, can smell war, are provoked to race by a voice, are called to battle by the sound of the trumpet, grieve when defeated and exult when victorious. Some horses can recognize an enemy in battle and attack by biting. Some recognize only their masters and will allow no other to ride them. Only the horse weeps and grieves for its dead or dying master. Mares are sometimes impregnated by the west wind. There are three kinds of horses: noble, good for battle and work; common, good for carrying burdens but not for riding; and hybrids, born from a mixture of two kinds.
Most sources have a long chapter on horses, which includes not only the attributes of the horse, but also examples of famous horses and tips on what makes a good horse.
Pliny the Elder [1st century CE] (Natural History, Book 8, 64-67): Several stories are told of horses that would let only their master ride them, who defended their rider in battle, or who grieved at the death of their master. Horses are very intelligent. They may live up to 50 years, but mares die sooner. The mare loves her young more than any other animal does. At birth, horses have on their foreheads a love-poison called horse-frenzy; this is the size of a dried fig and is black. If the mare fails to eat this immediately, she will not suckle her foal. Is someone takes it before the mare can eat it, the scent of it drives him into a sort of love-madness. Near the town of Lisbon, mares stand facing a west wind and conceive a foal from it; such colts are very swift but only live three years. (Book 10,83): To make a mare willing to mate with an ass, her mane must first be clipped; a mare with a long mane is too proud and high-spirited. After mating, mares run either north or south, depending on the sex of the foal they have conceived.
Augustine [5th century CE] (City of God, Book 21, chapter 5): In Cappadocia the mares are impregnated by the wind, and their foals live only three years.
Aelianus [170-230 CE] (On the Characteristics of Animals, Book 2, chapter 10): The Horse is generally speaking a proud creature, the reason being that his size, his speed, his tall neck, the suppleness of his limbs, and the clang of his hooves make him insolent and vain. But it is chiefly a mare with a long mane that is so full of airs and graces. For instance, she scorns to be covered by an ass, but is glad to mate with a horse, regarding herself as only fit for the greatest of her kind. Accordingly those who wish to have mules born, knowing this characteristic, clip the mare's mane in a haphazard fashion anyhow, and then put asses to her. Though ashamed at first, she admits her present ignoble mate. [Book 3, chapter17] The mare also knows that with the birth of a foal she is producing love-spells; and that is why the moment the foal is born, the mare bites off the piece of flesh on its forehead. Men call it "mare's-frenzy". And wizards maintain that such things produce and excite impulses to unrestrained sexual intercourse and a lecherous passion. - [Scholfield translation]
Isidore of Seville [7th century CE] (Etymologies, Book 12, 1:41-59): Horses exult in fields, can smell war, and are roused to battle by the sound of the trumpet; when provoked by a voice to race, the exult when they win but grieve when they lose. Some horses recognize enemies and attack them by biting. They recognize their own masters, and some will not allow anyone else to ride them. They weep for dead or dying masters, being the only animal to do so. [Isidore continues with tips on what makes a good horse and describes their various colors.] There are three kinds of horse: one is noble and good for war and work; the second is common and good only for carrying burdens, not for riding; and the third is a hybrid of the first two.