I was walking along Rt 67 in Roxbury when I spied what initially I considered to be a large grey/tan dog walking at a distance across someone's backyard, then disappear behind their house. While I first categorized it as a dog, my brain told me what I was looking at didn't wholly fit into that idea. It was huge and the animal moved much more gracefully than a dog, and covered more ground than a dog would for that size. I waited to see if it would emerge from behind the other side of the house, but then saw something move through the woods, although I couldn't discern its shape at that point, as it was shrouded by the thick of trees. It occured to me it was a mountain lion, but I can't be 100% certain. Truly, it was its size and how easily it moved, like the difference between watching a man walk and a woman walk.