I had a horse that would rear on a totally slack rein. It was her way of throwing a temper tantrum when she refused to leave the property (she was terribly barn sour).
That is the one thing that scares me really bad- rearing. I tried working with her for about a month but she had my number. Then I spent another month trying to sell her. Finally a horse trader got a hold of her. At that point (and I am anti-slaughter mind you) I was so frustrated with her, I didn't care where she ended up. Worst horse I ever had. I learned a lot after that one. Like trying a horse out thoroughly before you buy it. And she gave me a fear of horses I never had before I got her. Man, that brings back some bad memories.
Now, I had another horse, and Arab gelding, that would sometimes rear if he wanted to go forward and you were holding him back. I learned to work with his energy and kept him moving when he was in that "mood" and the rearing never scared me or got out of hand. I never felt like the rearing was directed at me. He was just like a hyper active kid. Instead of trying to make him stand still, I would give him slack rein and turn him in circles. So I knew how to prevent it, and we got along great. As a matter of fact, I considered him a really fun horse.
Now the first horse, the mare, I felt really threatened by, because she was doing it as a temper tantrum to get out of working. I felt it was done to intimidate me. And it worked.