Bad ass
I would have thought this just another internet urban legend - were it not for the pictures...
A couple from Montana were out riding on the range, he with his rifle and she (fortunately) with her camera. Their dogs always followed them, but on this occasion a Mountain Lion decided that he wanted to stalk the dogs (you'll see the dogs in the background watching). Very, very bad decision...
The hunter got off the mule with his rifle and decided to shoot in the air to scare away the lion, but before he could get off a shot the lion charged in and decided he wanted a piece of those dogs. With that, the mule took off and decided he wanted a piece of that lion. That's when all hell broke loose... for the lion.
As the lion approached the dogs the mule snatched him up by the tail and started whirling him around. Banging its head on the ground on every pass. Then he dropped it, stomped on it and held it to the ground by the throat. The mule then got down on his knees and bit the thing all over a couple of dozen times to make sure it was dead, than whipped it into the air again, walked back over to the couple (that were stunned in silence) and stood there ready to continue his ride... as if nothing had just happened.
Fortunately even though the hunter didn't get off a shot, his wife got off these four...
[Addendum: According to Snopes (Urban Legends Central), Berry (the mule) has a felinoid chip on its shoulder, that this is not its first encounter with cats, and "...the mule grew more aggressive over time in his pursuit of mountain lions..."]