Aww, cute puppy. I didn't realize that Mini Pins had underbites, too. I've got a Shih-tzu and a Japanese Chin and both have natural underbites, due to the shape of the face. I wonder how many other dogs have natural underbites?
I always crack up when I look at my Japanese Chin. He looks like he is smiling with ill fitting false teeth on the bottom. You never see the top teeth.