I vote for cougar as well. They are very elusive. I have lived in cougar territory my whole life and only seen one. I hear them quite a bit. Our govt authority denied for years they were here. So, yes, keep the cougar idea out there. Especially if the attack was at night. Cougars jump from the trees or a ledge. They get purchase on the animal close to the neck and their kill method is to break the neck if possible or cause bleed out from a major artery.
I would think that the cat jumped the horse and the horse realized just in time what happened and took off running before the cat got hold. If she kicked the cat on the way out of there, she may have stunned it enough to get back to the main herd. The cat would have had enough and would not venture into the herd at that point.
Someone suggested a bear. I'd nix that idea. A bear is not going to go for the back of an animal. They attack from the front or side. Never from the top.
I just looked around on the 'net and cougars are definitely in Texas.