I won't need one... by GSo I thought. I tend not to ride my horse at anything faster than a walk bareback, due to her TB spine (ouch). So when my husband and I hopped on bareback for a few minutes just to spend some time with our horses, neither of us grabbed a helmet (one of the few times).
My horse stepped over a ground pole--almost. She kicked it, started, backed up, and the pole caught in her pastern. I managed to stay on long enough to help her figure out how to get free of it.... and on the long, long way down (it always takes about 20 minutes to fall, have you ever noticed that?) all that I could think was: "My husband is right over there. If I land on my head without a helmet he is never going to forgive me!"
I'm really not sure how I managed to avoid landing on my head. Next time, I might not. My horse is always terribly embarrassed when I fall off and does her best to avoid it, but accidents happen.