I guess you are probably right. It must just be an optical illusion to me because that tail looks way to long to me for a deer, but it might just be the angle that is throwing me off.
Looks a lot like a golden retriever/lab mix to me. However, could be a deer, just looks a little too uniform and not "splotchy" enough to be losing its summer coat. I am not an expert though!
That is a deers backside. The tail looks long simply because it is flicked to the side towards the camera.