I have seen the occasional deer on my early AM rides. The ones that sort of live in my neighborhood have a very predictable pattern and always cross the road at the same spot, or go across my driveway in the same spot. I do not go fast at this time, even if if I am going downhill, so I can easily stop.
The squirrels, etc., well I have those all of the time.
And yes, just adding in one ride a week for 12-15 miles in the dark, adds up. I've had 3 headlights, and the one I have now is so bright that drivers often flash their lights at me!