If you're going to ride a horse you're going to experience a spook. Every horse, no matter how bomb proof it is, has something that freaks it out. It's just in their nature. In my experience there are three kinds of spook: 1) the plant and stare, 2) the jump sideways and bolt, and 3) the spin. I call the third type a "dirty spook" because it's the one most likely to deposit me in the dirt.
I don't want to give anyone the impression that Coriander's introduction to trail riding has been completely calm and spook free, because it hasn't. He spent most of his life on 5 acres in southern Florida, now he's in NY surrounded by flora and fauna that he's never seen before and it freaks him out. This week, for instance, he planted and stared at a raccoon in a tree and a baby bunny, he also prompted me to dismount due to his body trembling fear of paint horses (Interestingly, the first time he spooked under saddle was at a gray horse. Poor boy had never seen one before.). This kind of spooking doesn't bother me at all. It's mostly harmless and exactly what I'd expect from a horse that hasn't been anywhere or done anything before.
Today, though, he pulled out a dirty spook. We were about to turn off into a trail when we flushed a fawn and it took off into the woods. Coriander freaked and spun right, fast. Somehow I got my torso to the right even though my seat went left and I managed to stay on top. It was pretty close though. Good thing I didn't get dumped because he would have run off and learned a bad lesson. As it was I was able to get him to go past that spot and continue our ride, proving that spinning away from something he was afraid of wasn't the right answer.
The last time I rode a dirty spook I was still in high school. We had an off-track standardbred who had a nasty habit of spinning every time he spooked, and he spooked a lot. I decided to take him out one day to give him some exercise, since his usual rider had been too busy with work to get on him. I had taken him a couple miles down the road without a single spook until right before we got to the spot where I had previously decided to turn around I noticed a leaf skittering across the road. BAM! All of the sudden I was hanging in the air to the left of the saddle while he was booking it to the right. I tried valiantly to stay on but with my right foot hanging over the pommel it just wasn't going to happen. I then got a much closer look at that leaf in the road. Fortunately my Mom was home and came out looking for me when the horse arrived sans rider. Of course when I got back he was happily grazing on the lawn. I grabbed him up, got back on and we TROTTED back up the road. I have no idea whether that got the right message to him or not, but I was a teenager and I was pissed.
Anyway, I'm not going to pretend to be the world expert on spooking but I do know one thing- don't punish your horse for being afraid. Stay calm and patient when your horse gets upset and he will learn that the world isn't out to eat him.