Everything people have said is unfortunatly true.
You need to have a shutter speed of say 1/200 (minimum) to freeze jumping horses, 1/250 or 1/500 would be far better.
I've shot jumping dogs indoors, but never houses, though I'm going to say there are a few similarities.
1. Don't use autofocus. Manual focus on a jump, this is easy the house is going to come to you.
2. Set your shutter speed to 1/200, and your lens wide open (for your lens this is f/5.6). Turn on the extended ISO range. See what ISO the camera thinks you need.
If this is at your highest (12800) and it still wants more, see how under exposed the image will be. One stop is ok, you can fix that later (sort of).
You may need a faster lens. If I were you I would look for a manual focus 135mm lens. These are about what I found I needed for dogs (but I think with horses you are farther away) and they can be found for cheap. You can frame by moving to a different location.
I'd look for a copy of the Vivitar 135mm 2.8 or the Pentax-A 135mm 2.8. There is a 3rd party version of this lens with the electronics "A" setting for about $30 on the Bay and the Vivitar is even cheaper.
I'd get a lens with the Pentax-A setting as it will allow you to let the camera control all the exposure details. The Vivitar will save you some money, but isn't really as easy to use.
Both will produce better images than your current 50-200mm for this type of shot. (plus make a dandy portrait lens).
You can also look for a 100mm/105mm f/2.5 or 2.8 MACRO lens (except these will set you back alot more).
Hope this helps.
Good luck