I had a quite frightening experience with a bolting horse once. I was quite a beginner and had gotten to know this man who had some horses and gave lessons. He wasn't really interested in the teaching part but mostly just wanted company for trail riding. Well, one day my fianchè and I went for a trail riding with him and some other persons. I was on an Argentinian pony and my fiancè on an arab mare. Everything went well, I was happy.
Then halfway this horse guy told me to change horses with another of the group. I wasn't too happy but said okay. And mounted an arab-mix pony. My fiancè and I road side by side while the rest of the group went after us.
He said "Okay if we trot a bit?" Fine with me. So I squeezed my legs a bit and parted in trot but almost immediately the pony accelerated and started running. I lost one stirrup, the pony was now starting to gallop, and I lost the other stirrup. And so I started thinking "stay calm, stay on the horse" and I tried to figure out how to stop him. I had been told about the one-rein stop but we were on a small gravel road in the mountains, one of those that go up vertically on one side and down vertically on the other. The road went downhill and took a little turn and then uphill. And that pony just kept running!
I knew that the road would go up, then at a certain point take a sharp turn and go downhill very steeply, so I knew that I had to stop him before that. Finally when the road started going uphill he seemed a bit tired, and luckily on my right side the side of the hill flattened out a bit and I just took the chance to turn him right, but not too abrubtly as to not make us fall. As soon as he stopped I hopped off. He was still full of adrenaline (so was I) but I just held him tight in the reins and thought to myself "Okay, you didn't die, You are fine, just relax, and breeeeathe".
After quite a bit my fiancè came on the arab mare, he was sick worried and he had tried to hold her back to avoid racing with us. Fortunately he had succeded in it (otherwise I wouldn't be here to tell). He said that he had kept looking at my red helmet and after the little turn and the uphill part he had prayed to keep on seeing it, because that meant that I was still on the horse. And he just saw that little red dot becoming smaller and smaller (About 1 km from him).
After 5 minutes the rest of the group came and the horse guy sais casually "Everything all right?" He hadn't even noticed. I said "Well, this €%€& pony ran with me. Can I please get on the Argentinian pony again? Otherwise I'll walk". All right, no problem. The other girl was happy to change horses, God knows why. So I got on the other pony and we came home.
After that we stopped going trail riding with this guy.
Lesson: Never change horse halfway if you don't know the horses. This episode scared me off the trails for two years...