Cool picture. I think it's an owl judging the width of the tail - looks too broad for a bat. I think a bat's wings attach all the way to the tail; except a free tail bat, I guess. I wonder what spooked the deer because owls are completely silent in flight. Their feathers are designed in such a way that they don't create any noise which allows them to detect (hear) movement in complete darkness. I can attest to that personally. I was varmint calling one night years ago and I as stood up to take a look from the blind an extremely large owl was about a foot away from my face. I think it was as surprised as I was, but even as it frantically flapped to get away it was silent. I only felt the wind of it's wings. I have captured what I assume to be owls on my game cam, but the shutter is too slow for a clear photo. I can make out the eyes, though.