A reflection is caused when a light source bounces off of a surface. When light bounces off a smooth and shiny surface, it will reflect at the same angle it entered the surface. For that reflection to move, as in your video, either the source or reflecting surface has to move. Within the frame of the video, I can see nothing moving except the dog and you/the camera.
I have my doubts this is a reflection, but as I said, a reflection was my first thought. I do know dogs can see reflected light from a personal experience with my own dog many years ago when sunlight was reflecting off my watch crystal and onto the wall. I was using a screwdriver and the reflected sunlight was moving up and down the wall/ceiling. The dog, a very intelligent Shetland Sheepdog, sat mesmerized, her head moving in unison with the reflecting light as it moved up and down. It was very amusing at the time watching the dog's bewilderment.