I hit a dog with my unregistered car. Who should I tell?

Sorry to be so somber this early, but I just hit a dog on my way to work this morning. My car is unregistered (as of Oct. 31 2010), so I can't stick around. I stopped really quick and it was breathing heavy, but it wasn't moving. It was just lying on the side of the road in the grass. It's not a busy road road and he kind of looks like he's sleeping in a yard, so I doubt anyone else will stop or think anything is wrong. I'm now at my office. Should I call anyone? Should I just forget it ever happened? My front end is dented a little with blood on it, but police don't actually investigate this stuff, right? Should I go back and stab it in his head to end it's suffering? What are the penalties if someone saw me and got my plate number? Probably just a stupid fine, no? The dog was pretty old and probably wasn't going to live much longer anyway. Please advise.