We found his weak spot....
My daughter is 9, and I would say intermediate beginner. She has a 5 year old quarter pony. He's young, but he's very well trained and quiet. They are doing great riding at home, at the trainer's, and at the 4H advisor's house. Normally not spooky at all. Motorcycles, truck air brakes, machinery, takes it all in stride. But one thing gets him. Another horse cantering up behind him, or seeing other horses running in close proximity.
Last night we had a riding practice at the fairgrounds (fair is in a month). There are 2 arenas, separated by a pavilion. Someone started running barrels in the other arena, and he had a little buckfest. Kiddo hung on. She went around 2 more times, but he was still extremely worried about the horses in the next ring, and had decided that he was afraid of a large saddlebred that another kid was cantering in our ring. He was obviously getting ready to buck again, so we put one of our advanced riders on him. He bucked with her twice, but she was able to push him through it and they rode for about 15 minutes. After that we did showmanship in the center of the ring while another group (horses he didn't know) rode on the rail, many of them cantering. He had settled down by then and did his ground work perfectly, but that may just have been because he was tired.
Is this a common problem? Any suggestions?