Fat And Thin Dogs

Riddle: Two dogs are sitting on a porch - a fat dog and a thin dog. The little dog is the son of the fat dog, but the fat dog is not the father of the thin dog. Can you explain?

Answer: The fat dog is the mother. Source: https://www.riddles.com/772

Two dogs are sitting on a porch - a fat dog and a thin dog. The little dog is the son of the fat dog, but the fat dog is not the father of the thin dog. Can you explain? — Fat and thin dogs by Anonymous Riddle