If it was a dog it would be very large; perhaps a doberman or rotweiller. I would recommend next time you go out there to go back to the same spot (if you remember where it was) and see if there is a darkened tree or tree stump. Would be very interesting to see another pic at the same angle without the thing there, proving more that it is something alive, or an up close pic of what it really is to help clear it up.