Dog. Wait there and shoot it next time it comes by. It's likely running your deer, as someone else mentioned. That's if you are lucky. Big dogs like that can get a taste for deer and will chase them down and kill them. People's pets need to be kept under their control.
I don't shoot things I won't eat, unless it a yote, or a terrorist, or Robots, oh yeah, I'd definitely shoot a Zombie. Those things are crazy
deffinatly a great dane huge barrel chest and long skinny tale. grey hounds are more slender in the mid section.
TJF nailed It. Definitely a dog but I can guarantee you that's not a black lab. Way to small of an ass end and that tail Is all wrong for a lab.