William Steig 
Doctor De Soto 

Doctor de Soto, a dentist who also happens to be a mouse, is the best dentist in town. He treats animals of all shapes and sizes – that is, except for "cats and other dangerous animals". When a fox appears in his surgery with bad toothache, kind-hearted Doctor de Soto agrees to help. But is he foolish to trust a fox? Could a small mouse outwit the foxiest of animals? This modern classic is full of mischief, warmth and humour.