Seeing Double

I must of been around 13 at the time.
Just sitting in the living room and felt something weird. I looked to the rats cage and saw a white rat leaning up on the glass.
Now our rat at the time was white but something about the one I was seeing seemed odd.
My brother was looking out the window and my dad was doing something at the other end of the room. I stood up and very slowly started walking towards the cage.
The rat vanished and I walked faster. When I got tot he cage I looked in and saw the rat curled up and sleeping in the corner. It wasn't him I saw.
When I thought back about the rat I saw I realized that everything about when was white and their eyes were slightly see through.
This was the first time I saw her. She was the rat we had before him and it was her old cage that he lived in.