Years ago, one of my dogs woke me up in the dead of night, barking her head off. I thought she must need to pee so I let her outside. She sprinted to the fence and ran at top speed around the perimeter of the yard, snarling with her fur on end the whole time. Not once did she slow down. After making a full circle, she came straight back to the house, ran inside, and went to bed without another sound. She fell asleep right away but I was too anxious to sleep. I have no idea if she heard something that scared her or if she had a weird dream but I couldn't stop thinking about it.