This happened a year ago in the winter time.
It was about 10:30pm and dark outside. I was staying in cabin on the lakes by myself. I had all the lights on in the cabin and a fire going in the fire place. It started to get hot in the cabin so I opened the sliding glass doors to get some cool air in. I let my cat outside hour before I opened the door.
I was at my computer with my back towards the door when I heard my cat come flying in the cabin, so I got up to see what was going on. I looked over to my right to see my cat was on top of my china cabinet. I was standing about 6 feet from my sliding glass doors that were open. I looked straight ahead to see this black smoky mass come through the doors. It was about 4 feet long 2 feet high. It was off the carpet about 6 inches, with a black rolling smoke for its body, whatever it was, it stopped as soon as I saw it. I was standing about 5 feet from it; we were standing there looking at each other for about 8 seconds when it moved like a worm back out the door and around the side of the cabin and disappeared in to the night. I just stood there for couple minutes in shock trying to figure out what I had seen, I didn't feel scared just in shock...
Weeks before this my roommate and myself could hear something moving around up in the empty apartment above us, we went up there a few times not to see anything out of place. Few days after I seen that black smoky mass my cat disappeared, never to see him again. My roommate moved out days later, I stayed there for couple months never to see or hear anything again.