Travel delays: Just as when your car won't start, your hero can be disastrously or fortuitously delayed or stranded due to:
Death and injury: Both horse and rider are flesh-and-blood creatures, not machines. If the horse spooks and dumps the rider, any number of bad injuries can occur, from broken bones to broken skulls requiring lengthy delays or alternate forms of transport. It is also a fine excuse to introduce new characters to attend the hero.
- Illness: Colic is common when feed is irregular or strange/moldy or the horse doesn't get enough water. Horses also catch colds, and certain diseases can run like wildfire through a whole stable overnight. A sneezing, coughing horse might be treated like a leper by a prudent innkeeper.
- Soreness: Unfit horses get sore backs, saddle sores, and go lame just like unfit people want to die the morning after they start their new exercise regimen. A kind rider will take this into account; an uncaring rider will make the problem worse and discover himself stranded on the trail instead of in town where he could have found himself a different mount.
- Lameness: In rocky country a horse can easily stone-bruise a hoof, twist an ankle among the rocks, or pull off a shoe. Really bad trails or rutted roads might cause stumbling, falls, and injury, especially at higher speeds (which is all that Hollywood ever shows. When's the last time you saw the hero walking his horse anywhere?)
- Shoes: Throwing a shoe is a perfectly legitimate reason to delay a traveler somewhere. If the shoe doesn't come off cleanly it can twist on the hoof, leaving the poor beast hobbling along with the equivalent of a broken high heel nailed to his foot. The caring rider will end up leading the horse if he values it, and look immediately for the nearest blacksmith. This, of course, slows the chase/flight considerably!
- Stupidity: Your rider failed to tie up his horse properly, let it eat too much/too little, let it eat poisonous but yummy plants, or fell asleep with the reins in his hand and woke up to find the horse departed for parts unknown.
- Bad roads/weather: Rider is too stupid/arrogant/impatient to wait out the darkness, ice, driving rain and lames the horse, falls off a cliff, drowns in the river, etc. (Or causes poor horsey to do the same.)
- Horse takes exception to unexpected boogeyman (bear, llama, cow, birds, charging warriors, anything strange-looking arising under its nose) and bolts over a cliff, off the road, through the trees, scraping rider off in the process. This is also a good way to get your rider lost, with consequent plot merriment.
I love Charles M. Russell's work because he always got it right. I've been on trails just like this, and had/seen unexpected stuff happen that instantly induced terror. This painting says it all. The bear, the rocks, the panicky horses, the steep drop... ick.
- Horse falls with rider: slick cobblestones, ice, rocky footing. Half a ton of horse landing on your leg hurts. Trust me.
- Horse is only half-broken to start with. Unwary rider buys the beautiful beastie and discovers to his sorrow the critter bucks/bites/kicks/throws itself over backward/bolts or otherwise is unmanageable. This also works for the hero who tries to put an improperly-trained horse between the shafts to pull his wheeled conveyance. The wreck in this case is that much worse. There is no stronger bit of advice in horse trading than caveat emptor.
Economic distress: What happens when your horse/donkey/mule is your only means of earning a living? Who does this affect?Many writers don't consider the "horse in the background" quietly plowing the field, packing goods to market in its panniers, or nursing foals in the fields that will go on to fuel the economy of their world. But remove them from the scene and what happens? Your traveler gets stranded, your commerce grinds to a halt, and war becomes a matter of foot soldiers with entirely different tactics, moving at a whole different pace. Even the lowly donkey is an economic driver in some parts of the world, putting its lucky owner one up on the guy who doesn't own one. Will said owner hotly pursue some jerk stealing it? Probably. Personally, I would laugh very hard at the notion of a hero stealing a horse unopposed and riding merrily away. There's a reason horse-stealing was a hanging offense in the Old West.
- Farmers whose fields don't get plowed and have to wait until the kind neighbor is done with his own, which puts him late into the fields, or he struggles to pull his own plow because he can't afford a replacement. Will his family starve?
- Carters who can't move their merchandise without a beast between the shafts. Suppose some illness ran through all the local equines? It would certainly make oxen very expensive. What is the downstream ripple effect of this guy not making his rounds?
- Knights errant, mercenaries, cavalry of all types. Running off the enemies' stock is a great way to tip the scales in your favor, but overdone as a plot device. The sheer economic disaster of losing half the remounts can put a serious crimp in a campaign. If the ships carrying your horses to war or on some exploration of the New World sink, then what? (Aside from the sheer ghastliness of trapped horses going down with the ship.)
Horses are perfectly capable of causing massive plot mayhem all by themselves without anyone laying a hand on them, bad guys charging from ambush, or other unlikely meetings of human beings at awkward times. It is really satisfying when a writer uses a horse's natural proclivities to inconvenience his hero or put one over on the enemy. Hopefully this has given you a few ideas.
'Til next time!