I have been giving a lot of thought to animal reactions toward the paranormal on cases lately. If an animal flat refuses to go in a certain room and acts strange in the doorway of said room that's one thing. But animals do things we just don't get sometimes. And more importantly, their senses are a lot more attuned than humans. I believe sometimes when our pets react strangely to something we can't see, there is a more rational explanation. It could be an emergency vehicle streets away that we can not hear. Or the vibrations from a jet flying high overhead we are not aware of in our homes. We do not detect it, but aircraft still disturb the air, create vibrations, and make noise. Animals are capable of detecting this, and thus have a reaction to it. We think they are acting strange, or that something unknown is in our homes.

I would like to hear opinions on this.

I've seen cats watch things move that none of the humans in the room could see. Who knows?

Great topic Bobby, and I know of a few cases myself which animals have acted strange in Paranormal situations. My neighbor who has had various paranormal occurrences tends to notice her cat acting strangely when she feels a presence of something she would call "Spiritual" her cat seems to go to the source and stare at the ceiling. There are thousands of things to explain this but, this woman experiences a lot of activity. I wish I could witness it myself.

I've also heard often that dogs will bark and act hostile towards spirits, although those are mainly hearsay accounts.

My animals, really don't react any differently than normal. Either they don't notice, or don't care.

Very good topic BobbyT, being the animal lover that I am and working in the field as well I've heard many stories and seen firsthand behavior similar to what everyone else is sharing above.  But, with all off that I could still debate on and on, wheather a pet's abnormal behavior was caused by paranormal activity at the time or if there really was a rational explanation for it.
 
Just like wild animals after all its survival of the fittest, they have to be on constant guard for predators any wether changes that could affect their food supply, population, or habitat any little thing that could affect their well being.  After all what are the majority of our pets, but descendants of wild animals that were at one point domesticated.  I think we easily notice abnormal behavior like this in our pets because by some point or another we already recognize their body language, their otherwise normal behavior and routine.

But, who is to say that our animals can't sense or see entities like Hisoka had mentioned above its hard to say unless you're in their shoes (or paws  ) After all if and entity were to present it self in and ill willed manner wouldn't the pet's natural instincts tell it to be on guard or act in a defensive kind of way. 

Like I said I can go on and one about this,  I know one thing for sure if one morning I step out of my house and I see hundreds of rats, and birds all heading in one direction as if trying to escape, I will not hesitate to follow right behind them.