A dog's stool that is dry first and then loose may be caused by indigestion or improper diet. If this happens to your dog occasionally, there is no need to worry. You only need to help the dog adjust his diet. It is easier to feed him some. Digestible and light food can also be fed with appropriate probiotics to help regulate the gastrointestinal tract; if the dog continues to have diarrhea, it is best to go to the pet hospital for examination.