Sounds like a youngster? I had the privilege of caring for a lively 18 year old, he loved going out for a ride, it was the shining pinnacle of his day and he stepped out with a lot of joy and pride.
One winter it rained for a week and we were not able to take our daily ride. When at last we saddled up, he was keen but soooo well-behaved, I was very proud of him. Until...
Unseen to both of us there was a rabbit in our path under a clump of grass and he almost trod on it. As the rabbit shot off fast as it could, up went the old horse, standing tall on back legs! Down and boom: a huge twisty buck and a we were galloping off without rhyme or reason. I knew the land, so I just stayed with him and tried to keep us on a safe track. We eventually eased down to a walk then to a snorty standstill, but we had gone a long way by then! It took about 45 minutes to walk home. How I stayed on through it all I do not know and although I was riding fit, I had torn muscles the next day, as had he, the usually very gentle giant.
Actually, the whole adventure was rather invigorating