Ghost stories have been doing the rounds since times immemorial, and aren't confined solely to human apparitions - it would appear that there are any number of spectral dogs too.
Some are based on folk memories of long ago, or are attempts to explain natural phenomena not properly understood at the time: other hauntings were intentionally invented by smugglers to discourage closer investigation of any strange lights or sounds. Other incidents might be attributed to wishful thinking or a trick of the light: but even when all these are discounted, there are many encounters that seem to have no obvious or straightforward explanation.
Some years ago, while staying overnight at a friend's house, I ended up sharing the bed with an invisible but very determined phantom hound. Every time I dropped off to sleep I would be woken some time later by the unmistakeable feel of a dog's body pressed against my legs and pushing me towards the edge of the bed. Switching the light on revealed that nothing was there: but the moment I dozed off again, the whole performance was repeated. In the end I resisted the pushing, and having made my point, it stopped and simply became a comforting pressure and I finally managed a few hours of undisturbed slumber.
In the morning I mentioned this strange experience to my hosts. It turned out that it was a fairly regular occurence. In the early houres of the morning they too would often be awoken by the ghostly dog pushing their bedroom door - always left slightly ajar - further open: having greeted them, they would then hear the click-click-click of his toenails as he made his way down the uncarpeted stairs and headed off in the direction of the garden where he presumably then spent the rest of the day.
I've thought many times since then about my invisible canine bedfellow: was it really the ghost of a dog who had once lived there? Or a very vivid and reoccurring waking dream during the night?
I still can't quite decide.