re: Would a mountain lion endanger someone riding a horse?

I wouldn't worry as much about the lion taking you off the horse (unless he had some height and really good access to you on the trail), as much as spooking the horse and you get thrown.

For the most part, they will stay away. I know in California they have ~15 or so verified attacks over the past 25 years, including a couple of fatal attacks on adults, but even in California where the numbers of lions, population density and proximity make casual encounters (probably the most dangerous from the human side) more likely, this seems to be extraordinarily rare.