What you are asking is tricky to do, and it depends on the detail of the background you want to insert into.
Sometimes you can clone a separate section of the background, insert the clone into the hole you want to fill, and then slightly manipulate the clone so that it blends well and looks unique (or different) from other sections of the background.
For example, a mildly cloudy sky or a grass lawn works well for this and sometimes doesn't need much manipulation aside from blending.
This doesn't always work though. It does depend on the image you are working with, but trying to clone a separate section of the fence to fill in the area behind the horse may not work depending on different lighting, changes in angle of the fence, etc.
I also recommend providing a link for the image. And others will be able to offer more and better advice.
(When I use the term clone, I'm just referring to the copy created when I copy the selection and then paste it in. I'm not referring to the clone tool)
Ok, so it isn't just me. I was wondering the same thing. Thanks Dan.