Probably wouldn't hurt to get yourself a high powered spotlight of sorts, so if you hear people in the barn you hit the switch and flood the barn with god alone knows how many watts of illumination. Firstly it'll give you a clearer image of what's going on, and secondly it'll screw their night-vision and obscure you, thus helping to increase your ambush options.
Just watch it with whatever you choose that it doesn't wind up hitting one of the horses, I'd wager that they're not cheap, in fact some horses are more expensive than firearms (and if it's a Shetland pony, probably more deadly).
Heck, for that pure Ranch look, get a Winchester repeating rifle.
