For what it is worth, here is what I see. I'm reasonably sure the "cat" was already dead.

Certainly a mule could perform the actions that we are seeing in the photo, no question about the ability of the animal to do that. Having horses only enforces that opinion. However, a healthy mountain lion would not allow itself to be put in this position, further, and even more incriminating is the body language of the hounds. If there was any type of fight left in this cat those dogs would be all over that feline like a cheap suit! One of them is even sitting down, certainly an inidicator that the battle has been over at least briefly.

Notice the empty scabbard and no evidence of a bow in the saddle either. That is because the hunters have whatever weapons they used to kill the cat, in their possession. I can guarantee you that the owner of this mule would never stand around and take pictures while his valuable piece of property (the mule) is being attacked and threatened by serious injury or death, not happening, no way. A well trained mule or horse is very valuable. You don't just train them overnight, it takes lots of time, effort, and feed-all of which comes at a price. No one is just going to stand by and let an investment of that magnitude be put in harms way- not when they have weapons in their hands.