Behind the Wall

Apparently we have new residents occupying one of the houses behind us. I can't say we have new neighbors, because here in southern CA we separate ourselves from each other with 7-foot-high block walls. I hate that. Anyway, my sweet dog Nikki alerted us to the presence of a new dog nearby. Now, Nikki doesn't bark unless she's playing with us (the only way you'd ever know she's upset is that a line of fur stands up along the ridge of her back), but she's always intrigued by a new dog and the chance to make a new friend and sniff a new, um, smell. She's the most unassuming dog you've ever met:
The dog behind the wall (sounds like a horror movie title, doesn't it?) made some noticeable commotion every time it sensed Nikki nearby. Because I'm the curious sort, I thought I would peek over the wall and see what was beyond. As I said, the wall is taller than I am, and I didn't want to bring out a stepladder - and look like a burglar if the mystery dog's owner happened to be there. So I held my phone over the top of the wall and snapped a picture. This is what I got:
Doesn't look so bad, right? I mean, at least its tail is wagging (I think), and that's probably a friendly bark. Well, that's not the end of the story. A week or so later, the dog behind the wall took on a more sinister sound. It started jumping up and trying to peek over our wall. As it got better at jumping, I began to see a nose appear above the wall for about .12 seconds every 5 seconds or so. And something made my blood run cold. The dog wasn't white any more. Using the my ingenious high-tech spying method, I obtained the following picture:

Now, let us consider the possibilities. 1) Dark Dog killed and ate White Dog - it certainly sounds vicious enough; 2) White Dog ran away when Dark Dog showed up and began bullying her; 3) The residents of the house thought that White Dog was not mean enough for them, so they got Dark Dog (note that this could be combined with #1); or 4) White Dog actually became Dark Dog after having an emotionally traumatic and physically transforming experience involving radiation or noxious chemicals.

None of these options make me feel good. But I'm gonna keep an eye on things, because Dark Dog Behind the Wall seems to be gaining strength in his leaping muscles - his whole head now appears above the wall. So if I don't show up for something someday, please come check my back yard for little pieces of me and my unassuming dog Nikki.