It looks unnatural in that, as you say, it's a rough and ready (and therefore very evident) selection. All down the horses left side of the face (ie our right), is now blurry and out of touch with the rest of the face, and the ears look like a rough cut.
However, I like the background though, much better.
One thing I would suggest, regardless of the cutting, is crop the dead area down the right hand side, or at least some of it. It's making it a bit unbalanced for me
