A person, dog, or cat, who is overweight due to overeating or lack of exercise. Generally, the extra weight will be centered around the middle (or gut), thus creating a rotund appearance.
For humans: I can't find a dress shirt that doesn't look like a tent. It seems like all these dress shirts are really made for Fatty McFattersteins.
For dogs/cats: My cat lives for breakfast and dinner and she has an extra pouch hanging down. What a little Fatty McFatterstein.