Horse 101 for Writers
Irritating mistakes writers (un-published and published) make regarding horses:
1.    The Everyready Bunny, er, Horse--
Horse are living, breathing, eating, sleeping, pooping, peeing animals. Emphasis on the eating.
The average-sized horse--1200 pounds--requires somewhere between fifteen to twenty pounds of food a day. At least. My little Arabian gets by on the low end of the scale, but a big, burly quarterhorse, working on a cattle ranch is going to need the twenty pounds and then some.
Modern horses are expected to carry enough weight to cover their bones well. Some argue that American horses are on the plump side. Horses in some Third-world countries work a lot harder and are fed much less. (They are typically much smaller, too.) Nevertheless, it's pretty ridiculous to expect a thousand pound animal to run endlessly, day and night without food and water.
Horses, despite their size, are fragile animals. The slightest change in food can send some into life threatening colic. Their hooves must be kept clean and trimmed--hooves are basically fingernails. For a horse that is accustomed to wearing shoes, the loss of a shoe can be nearly crippling. With horses, Murphy's Law operates to the extreme. If they can get hurt, they will.
2.    The Fearless Horse--
Horses are prey animals. While the average horse has little to fear from predators, their genetic programming tells them that the world is out to get them.
I love the Lord of the Rings movies, but the sight of the Rohirrim's horses charging fearlessly toward the Mumakil (Oliphants) always induces a chuckle. Horse on the set of LOTR had to be acclimated to the various real threats they would face--knights in clanking armour, guys in scary orc suits, etc. The Mumakil were computer generated, which is fortunate, because your average horse would have lost his tiny little mind at the sight of thirty-foot, multi-tusked elephants.
Ditto any horse who faces dragons or any other fearsome beastie. Horses are chicken shits by nature. My horse thinks trash cans are monsters.
3.    Horseriding is Easy, No Skill Involved--
The horses at rental stables are generally the oldest and mellowest of the lot. That's why they are at such places. Just because you managed to stay on the back of old Trigger at The Ponderosa Dude Ranch doesn't mean you can ride a horse.
Peter, your hero who's never ridden a horse in his life, cannot simply leap on the back of a snorting war horse and ride to battle. Snorting War Horse is much smarter than Peter (trust me), and he can spot a greenhorn instantly. Leaping on a snorting war horse's back is a good way to get launched to the moon. (Or killed).
Horseriding is dangerous, even when you know what you're doing. Think Christopher Reeves. Even the pros get hurt, sometimes badly. A couple of riding accidents as a child left me with concussion-induced dyscalcula.
Riding requires muscles that most folks don't use everyday. If Peter, the Greenhorn, rides a horse for six-to-eight hours, his aches will have aches. He'll barely be able to walk. Heroism will be out of the question.
4.    Jiggling/Shaking the Reins Means "Go"--
This mistake makes me stop reading. And it's perpetuated by published writers!
A "cue" is anything that is used to communicate a command to an animal. With horses, it is usually verbal-- e.g., "Whoa"--or physical--e.g., a tap with the whip. An animal is trained when he responds to a cue--offers the correct behavior--consistently. (I.e., your dog, who only sits when he feels like it, isn't trained.)
Technically, you can use any cue. You can train a dog to sit using the cue, "Bark." A horse can be trained to speed up when you say, "Whoa." But, obviously, not every cue is practical.
Rein shaking is not practical. (By the way, people driving cart horses do "appear" to shake the reins to signal "go." They are actually slapping the reins on the horse's rump.) In most riding traditions, the idea is to use the most subtle cues possible. Riders in dressage, reining, and cutting strive to appear as one with the horse. There's nothing subtle about flapping the reins.
Typically, the reins are used to give directional cues--left, right--and control speed--slow down. The "go" cue is a tap or squeeze of the rider's heels to the horse's sides.(Experienced riders know this is simplification, but it will do for the scope of this article.) This configuration makes it easy to give two different commands simultaneously--e.g., speed up and turn left. The rider squeezes the horse with her heels, while using the reins to indicate the direction of motion.
Obviously, it gets complicated if the "go" cue is moved to the reins. What's the rider to do? Flap the right rein and pull on the left rein? What about riders who hold the reins in one hand? Are they supposed to alternately flap and then pull the reins? Not very practical in the heat of battle, huh?
Which is why the notion of flapping the reins is just so ludicrous.