You have a lot going on in that corner of the room. Stick to a solid colored rug to tone it down a bit, unless you're willing to change up your decor significantly. In terms of color, go with light - you have a lot of dark elements in the room already, and since the bed is pushed up against a wall, I'm assuming the room isn't very large, and a dark rug will make it look smaller.

I was going to say a solid, light-colored rug too...I'd go with a neutral that will match with the horse picture.