Often, dogs have soft stools or diarrhea, or they strain and are constipated. This problem can be fixed by simply adding fiber to their diet, such as pumpkin, or in changing the food they are eating (always make changes slowly). High quality foods are available in pet supply stores or online, rather than the local supermarket or grocery store, but homemade raw food is the best for your dog or cat.
I recommend the book, Dr. Richard Pitcairn's Complete Guide to Natural Health for Dogs & Cats (the current book is more vegan-oriented than his older book) for healthy easy recipes you can learn to make in an hour or so. The book has lots of helpful, enlightening information as well. I have been using his recipes since 1998 with good results in my cats over the years - shiny, soft fur and bright eyes, and increased energy and health into old age. (Dr. Pitcairn and his co-author wife, Susan, have long been the trusted name in holistic veterinary care and continue to be at the forefront of natural pet health; he is founder of the Academy of Veterinary Homeopathy.)