I believe that animals are able to sense energy patterns and some of these

energy patterns are such that they have a visible form to them that is not visible to some human sight.  Some of the ghosts have an odor that gives them away.
Animals are truly One with Nature.  They know how to hunt and take care of  themselves.  When we domesticate them, they do not lose the ability to sense and read energy around them.  Since people often do not sense these things our animal friends try to warn us of dangers and changes in the energy  patterns around us.
Animals can see and sense spirit more easily because they dont have what we call mind clutter to over shadow their senses. They rely very heavily on using all their senses. They also have no human doubt which is one of the main reasons they can see them. I hope this answers your questions.