Thanks for the different theories/advice guys, sorry my reply was so quick this morning. I was on my way out of the farm right at that moment.
Normally I'd say that it definitely could of been a fence or tree branch, but we just replaced our fencing with electrical fence in that area that is very safe- and it honestly doesnt look at all like a branch wound. I'd take pictures for you all but my camera has bitten the dust (literally. I dropped it in the dirt and got dust in the lense). They're almost perfectly symetrical, there are two sets of lines about two feet apart from eachother (on each side of her back and hindquarters like it jumped, slid, tried to hold on, and fell) with the cuts getting deeper near her butt and ending suddenly. I really don't see how a tree or bush could of done that.
I scouted the area and found a trampled spot with quite a bit of blood near the thicker part of our tree line, but unfortunately couldn't find more than half of a print because hoofprints are everywhere. Its definitely a cat of some sort, but I can't say much else.
I honestly don't feel like it could be a mountain lion because of where I'm located, but I made a few calls anyways and there have been the usual amount of bobcat (not uncommon cat in our area) sightings and a lot of coyotes, but nothing bigger than that. Still, as one of you pointed out, its strange that the cat just gave up like that unless one of the other herd members ran it off or something. We do have a donkey out there with them for that very reason but he has no signs of injury and neither do the rest of the horses.
The best theory so far, is probably bnayc's about the cat underestimating Xcia's size or thinking she was injured/ill. She's not exactly small, and the cat had easier (ish) access to miniatures, goats, cattle, chickens, or even ducks or turkeys. Why attack a 900 pound healthy mare?
We're going to see about borrowing some cameras to set up, but it will be hard considering how large the pasture is and the fact that it isn't all flat land. All but 3-4 acres are atleast lightly wooded. I'm especially worried for my mare, who is a 225 lb miniature and is due to drop her foal any time now :/ not sure what else to do though.
To give you an idea of Xcia's size, here's a picture of her. Not quite 'tiny and vulnerable' looking.