"We were shooting at a different place in the desert where there's these little bumps and things," Depp, who plays American Indian hero Tonto in the film, said on Thursday's edition of "Late Show with David Letterman."
"And so the horse that I was on decided to jump a couple of these little obstacles. The horse was unaware that the saddle I was wearing to sit on top of it was ... kind of faked ... to give the effect that I'm riding bareback," he recalled.
"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. All very calm for some reason, I figured that fear would kick in but it didn't. I was waiting," he laughed.
"All I saw in front of my eyes were these very muscular horse legs and striations of muscles moving. This kind of death machine. One word popped into my head: hooves. Hooves. In any case, mind the hooves."
Depp said he dropped off the horse in an attempt to escape being "horrifically mangled."
"I 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, he was coming straight down into the head," the actor said. "He just clipped me with his back legs."