Your camera sees all the white snow and thinks "this should be gray" so it underexposes the scene. If your dog was all the camera saw it would try to make him gray too, overexposing the scene.
The answer is to increase the exposure about 2 stops to make the snow look white and hopefully that will be enough to lighten up the dog too.
I have a black & white cat and afaik there is no digital camera that can successfully capture that kind of range in one exposure, so the long answer is we must decide what in the scene is important and expose for that, letting the rest go into over or underexposure.