When I was around 9 or 10 I was in girl scouts and once we went on a trail ride (so no helmets). These were really big horses but then again I was very small.
So the guy started to mount the girls on the horses and once every one was seated, the horse I was on started rearing. I was freaking out so I loosened the reigns a bit and leaned forward so that I wouldn't end up accidently hurtting the horse, and be some miracle I managed not to fall.
That day I learned two things 1. I LOVE horses!!! 2. always and I mean ALWAYS wear a helmet even on a trail ride( cause it just might save your life)!!!