Shed-barn. I prefer that the front wall be sheeted in, leaving a 4' opening if for a single horse. No door but hang a canvas tarps, old carpet, etc over the opening long enough to rub the ground. One should cover the opening plus a foot on each side, then slit up the middle. Another, 3' wide hangs over the slit. This allows the horse to easily move in and out yet keeps it dark enough that few flies enter. Plastic tarps are useless for various reasons. I like the 12 x 12 but without a solid door. Your horse won't go in it in winter but will prefer to bum up against a wall so he can see all around. Because of that be sure to set it far enough from a fence so he can use all sides.