I think my Pekingese sees dead people.
Without any warning, he will bark when nobody is there and there aren't any sounds.
Because of this I have deducted that he sees ghosts and he isn't very happy when they are wandering around unannounced. I'm not sure his bark is ferocious enough to scare them away but perhaps he feels like he should do something. After all, this is his apartment and supernatural entities should respect that fact.
Or maybe he's crazy and he hears voices. Or barking. Which he might understand better than voices.
I hope he's not mentally unstable. That would just suck.
Does my dog look nuts to you? Go ahead. Tell me the truth.