YES!!! A three hundred pound person can hurt a horse.
It depends on the size of the horse and its physical condition. I was pretty much raised on a horse I had three and was raised on a beef farm. I was on a horse everyday. My first job was at hightail stables where I mock the stalls and learn and help chain American saddlebreds. We had some champions and we had some horses that were from the blood of grand champions. They are very expensive horses. But I'd like to go riding I spend so many years but I don't want to be tied up following some trail with half a dozen horses in front of me I want to ride the horse the way it's made to be ridden.
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