Judging by all the people standing on their horses or crawling between their horse's legs in photographs, I think it might be. Either that or these people just have a death wish! I wouldn't think of doing dares that are that dangerous or stupid, though I know when I was a kid we used to dare each other to do certain things on horseback, like jump a creek, enter a class at a show that we'd never done before, or play games like follow the leader. Follow the leader could become quite a challenge when the most advanced rider took the lead!
Anyone who's interested in a good game of horse-related truth or dare, proceed to the games section. I have started a thread there!