I find that horses, knotheads as they are, tend to be very aware of the other animals around them.
I noticed the horses over here were hanging around the barn for a bit longer than usual so I went over to investigate.
I found a sick tom cat in the end stall with the one mare standing over him. It seemed as if the cat had distemper and would not respond to my poking it with the toe of my boot. It seemed to be in pain so I went and got the .22 and put it out of its misery.
It just seems like the horses knew the cat was sick.