I must have missed something here these pics have been bouncing around the internet for 10 years or more and each time the story is a little different. The link posted by Trip clearly states the lion was dead when the mule grabbed it! I think nitestalker was dead on when he said the countryside does not look like Montanna and the hounds would NEVER just sit there with a live lion thrashing around!!
The link posted by C3 states that while mules are effective against coyotes not so much against bears and lions.
Is it possible? Sure most anything is possible, is it probable? The pics in this thread are pretty much debunked by Snopes the description has nothing to do with reality the lion was dead!
Its a great little feel good story though kind of makes you want to hug a mule or even get one for Christmas