I didn't know his tail was tucked under, so I guess don't rule out squirrel then. Sorry, but rats are everywhere. I've never seen one in WI, but I hear people talk about wild rats around here. Mouse is ruled out at that size I guess. You'll just have to wait and see. I must say he does look like a teeny tiny chihuahua, but they are born with fur. Is there any other distinguishing feature? Does it have teeth? I can't tell in the picture, does it have fur colour starting to show or is it all pink?