Dogs have nightmares too

I haven't seen my dog be disturbed by a nightmare but I have heard that dogs also dream. One can tell if their legs are moving or sometimes if they're whining. It seems that it's the same with humans who go on about their days and then dream about their waking lives. A good friend of mine told me about her dog that she sometimes watches when it's dreaming. She said it's like in the dream, her puppy is running because of the movement of the legs.