I thought she was barking at the deer. As I stood at the back window watching the herd of deer down below, and a few of them taking notice of my American bulldog's deep-throat warning, she kept barking from the side porch off more to the North. The deer slowly started feeling spooked and eventually split off into two packs and trotted--one back to the woods, the other south down the river. My dog came in eventually but was hankering soon to go back outside. I let her out again and she just sat on the east porch giving a continual stream of warning barks.
I finally figured I better check what it was she was barking as and my wife peered through the trees and said, "That's not deer. I think those are horses."