Ah! Ghost stories like this really get my adrenaline pumping. They just freak me out - kind of a nervous excitement rush, I love it, lol.
I agree that it must be a spirit dog. I'm not really familiar with how one "acquires" a guardian like that, but they are here to comfort you and protect you. Normally a sign that something will happen, the closer the days come to that event, the more the appear so you know you're not alone (not necessarily a bad event...just a bigger, life-changing one).
I'm not sure if I ever really had encounters with ghosts. I think most of it as been in my mind, lol.
I remember once when I was really little, my mother was cleaning up a few things, and laying against the wall was a picture of my grandmother who unexpectedly passed away when I was two due to an accident. I was running up the stairs and saw the picture and just as I paused to look at it, I heard someone calling my name in a "cutesy way" and saying how beautiful I was and how they are so proud of me. Confused, I asked my mom is my grandma ever called me by that nickname before and she said yes, that that was her name for me...
A couple summers ago I lost my [first] dog. I was do depressed over her death and even blamed myself for not being there with her. For weeks after, every night when I would sleep, I would dream about her and she would always be energetic, playful, and so happy - almost as if she was telling me she still loved me and for me not to feel bad, that she is fine and I should be happy again.
The last "ghost encounter" of mine was sophomore year at college. The university is quite old and many buildings were used as hospitals and morgues for soldiers in WWII. There have always been stories of ghosts of the founders of the university, of children from town, and even of students who passed due to accidents or irresponsible behavior, etc. Well one night, I was drying my hair after taking a shower and how of the corner of my eye, I saw this short, blond-haired girl starring at me. Thinking it was one on my roommates, I started talking to her, but when she didn't answer I looked up at her to see what was wrong and she had vanished. Freaked out, I starting telling my roommates about what I thought I saw - they all told me I was nuts, but gradually they began seeing her too. I had brought a bushel of apples from home, and one morning when we work up, every single apple was carefully placed around the common room of our suite, almost decoratively organized... that was freaky. (There's a bit more to the story, but I don't want to hijack this thread.)