michael

The Haunting of Hill Horse

You know what they say, never look a gift horse in the third eye

Don't listen to Big Equine's lies! Finding teeth of unexpected shape or number is the best way to tell if your gift horse is carrying an unwanted spirit. By the time they exhibit external symptoms it may be too late.

according to nick cave, yes

They're very sensitive to everything around them and tend to react to it strongly, so it would be hard to tell whether a horse was actively haunted or just responding to regular things too subtle for humans to recognise. Functionally it makes no difference, of course, best to just assume they are.