Apparently, they were shooting a scene on bumpy terrain and his horse decided to “jump a couple of these little obstacles" and everything went "sideways."
"The horse was unaware that the saddle I was wearing to sit on top of it was jury-rigged, kind of faked ... to give the effect that I'm riding bareback. So basically it's not very tight on the horse. So when we came down, the saddle slipped and I went to the left and had the reins here and somehow had the wherewithal to grab the mane of the horse.”
Miraculously he remained calm. “I figured that fear would kick in but it didn't. I was waiting," he told Dave.
As he was being dragged a reported 25 feet he realized he must “mind the hooves.”
Eventually he had to choose between two options. "Will I go with the beast until someone wrangles it or will I drop? You know, it's a c**pshoot," the actor said, adding that he could have been "horrifically mangled at the very least."
Depp continued, "I dropped. It landed on my back, hit the deck pretty hard and the one thing that saved me was the horse's instinct to lift his front legs and go over me, step over me. Had he not, it was coming straight down into the head."
Letterman asked in disbelief, "Honest to God, where are the stunt people for heaven's sakes? You're Tonto! You don't need that!"
Johnny replied, "Well, that's the exact same question I asked.”
"The Lone Ranger," also starring Armie Hammer, opens in theaters on July 3.