Well, "had to" and "quick draw" may stretch it a bit, but...
I keep a .22 pistol (it was a Ruger MK 1 Target at the time) in a holster zip-tied to the front carrier on my ATV, which occasionally comes in handy as I work on the fences and pastures and such. I was leaving the barn and heading into one of the pastures one day, when I spotted a huge pine snake slithering away. Stopping the 4x4 and grabbing the pistol, I took a couple quick steps forward and stepped on the tail of the snake, stopping it before it could make it to some dense weeds. Immediately, the trapped snake turned back and struck at me, so I snapped off a hasty shot from the hip and put the bullet neatly through it's head. Of course, there was nobody around to witness the shot.