EVERY horse has the potential to run away. They have eyes and ears and process information in their own unique way. If you sit down and think about it - horses are dangerous - period. We do not control them with strength - it's respect and timing of cues.
I have been riding for over 50 years and no horse will run through me. Well in the beginning I had them run through me but not in the last 30 years. My seat, my strength, my technic will stop any horse.
That said I have rode run aways for others. Horses that just up and run and you can not get them to stop.
I can think of 2 really bad cases. Both knew how to get away with it. They just ran, wide open, sometimes straight at fences and the owners didn't have a clue how to stop it and both were going to sell.
I took a good western curb bit, one with good long shanks and welded a homemade port in the center, used a good strong tight curb chain and my good 3/4 inch leather reins with a good buckle on their ends. My headstall, my bit, my reins, equipment I would stake my life on.
I got on the horses using MY saddle and took them out into a big field and then kicked them into a dead run. After about 100 yards I leanded back, put my legs forward and yelled WHOA and loud as I could and then hit the horse with everything I had in my back and legs. I just about pulled the horse over backwards but they stopped and shook and I immediately released the reins.
Sure they shake but so be it. I then kick them into another flat out run and after another 100 yards sit back, brace my feet and yell WHOA and pull them just about over backwards, again not giving them a chance to stop on their own, hurting them as much as possible with the pull. They stop, they shake. I release all pressure.
I call for a 3rd run and after another 100 yards , sit back put my legs forward and yell WHOA BUT NO REINS.
They stop, they tremble, I praise them and walk them home.
They won't run again.
Who wins here?? Both horses were not sold and the owners never had a runaway problem again.
How did that harm the horses??? Soar mouth?? I don't care.. From that day on the word WHOA means something and they don't need a repeat of the lesson.
I did a similiar thing to one of my own 2 years ago and he got the lesson without all the running and slamming on. He just knew the minute I took ahold of him.