It's a rare horseman that hasn't had to deal with a spooky horse. Surviving the encounter can be an iffy proposition since we often don't know when that first fit of craziness is going to strike. It's easy to be caught off-guard and wind up off-horse.
Is it possible to teach a horse not to fear anything that it hasn't already encountered? The simple answer is no. Horses' fears are not very specific. They are prey animals--a fact which is beaten to death in discussions around the globe--which means they are self-protective. It doesn't mean they have to react to every little unexpected stimulus, but it does mean that we humans, with our predatory bravado intact, may not see things the way our equine buddies do. A leaf is a leaf is a leaf to a human. To a horse the size, shape, and movement of the leaf can make it something completely different.
The old measure of "sacking out" a horse still works, but the definition needs to be pared down to simply introducing scary stimuli over and over and over until the horse is less wary. It does not need to include beating or aggressively touching the horse with things that frighten it in the hope of driving out the fear demons. That doesn't work. That results in the horse fearing the human involved instead of seeing that human as a safe partner who can be trusted not to cause injury.
|Note the ears. They shriek,|
"You put... what is that on my head!"
The best approach, then, to getting rid of that little spooky brain hair you horse is harboring is to engage the horse in ways that encourage it to see you as a trustworthy ally. Starting as early in the horse's life as possible, gentle (but firm) handling is a great beginning. This doesn't mean that one must avoid alarming the horse. A too-stealthy, overly-cautious human breeds as much fear in the horse as that scary leaf. The horse is a body-language interpreter par excellence. It will see a human's cautious attitude as a signal that there is something in the environment that needs to be feared. A matter-of-fact approach will calm the horse and encourage trust. Not being afraid of the horse's fear is step number one. They won't get that they are the source of your fear. They will simply mirror it, assuming an outside cause that's going to kill both of you.
Introducing new stimuli doesn't have to be fear-inducing either. Bought a new brush? Let the horse see it, sniff it, then just go ahead and groom him with it. Clipping, bathing, loading on the horse-swallowing trailer...all of these are stimuli not found in nature, so each should be introduced calmly and patiently but with a firm "and now we're going to do this" (no matter how long it takes) approach. And new things should be introduced as often as they crop up. Don't shelter the horse, but be on hand to calm his fears.
Standing at the end of the lead yelling, "Whoa! WHOA! You're going to be okay!" is not calm. Standing next to the horse (at a safe distance) in silence, waiting for that moment when he looks to you for a cue is a better move. We humans are very bad at silence. If you screw this up, it's not the end of the world. It's the beginning of a very long reprogramming.
Just yesterday as I led my mare out of the field for a ride, I noted that she was a little skittish for reasons I couldn't pinpoint. It pays not to ignore a horse's mood, and I gave the area a quick once-over, looking and listening for odd things that I might have overlooked, but I didn't get upset. I didn't stop and ask her what was wrong. I wasn't overly-solicitous. I said, "Hey, girl! Let's go get some bug spray on!" And off we went, she tip-toeing, and I chattering about flies and dirt and what-have-you without really looking at her. By the time we got to the barn, she was calmer...so naturally I reached overhead to turn on the fan and sent her into a minor attack of the heebie-jeebies. I laughed and apologized, but didn't panic. When a horse is upset, that's not the time for the human to reflect anxiety. It's the time for just moving forward. If the horse is putting you in danger, it's the time for you to change direction or even put the horse into a pen or stall for a few minute to work out its own issues. Trust me on this; you don't want to get into a discussion with an upset horse. But if the horse is just a little jittery, move it on along.
We continued down the aisle, and my thought was that this was probably going to be one of those wild-and-woolly rides since she was already wired for an explosion. So I opted not to clip her bridle path. She's not fond of that. Instead I fly-sprayed her head. She's not fond of that either, but her distaste for the spray can be ameliorated if , after the first reaction, I spray my hand and wipe it on. Surprisingly, she was better with the spray than she's ever been. Go figure!
I spent more time warming her up than usual and opted for more flat work at a faster speed, which seems counter-intuitive. But the goal was to give her the idea that all was well with me, with the air and the leaves and the environment as a whole. Running her around and jumping like crazy for a half hour proved that beyond a doubt. By the end of the active part of the ride, she was calm as could be and our ramble around the hay field turned into her wandering into places she'd been unwilling to visit until that moment. I dropped the reins, relaxed my back to sway with her movement, and she was relaxed and brave because she was mirroring me instead of the reverse.
It's not possible to introduce your horse to everything that could possibly cross its path. We can't imagine what things will trigger a reaction, nor can we account for the sudden appearance of, say, a stray feral hog on the road. It is possible for the human involved to avoid having an excessive fear reaction. Gain the horse's trust, then be the trustworthy leader so confidence can be built. It's not a perfect solution, but it's workable and will help minimize (not eliminate) the danger of a catastrophic spook.
UPDATE: Here's another article on the subject of spookiness and how to keep it from becoming dangerous.