Like it or not, your dog will change as he grows, and he or she might develop some physical limitations that make using certain backpacks for dogs unfeasible. For example, if your dog has suffered from back problems or hip dysplasia, using a saddle bag that is designed to sit low on the back or the near the hip area is probably not a good idea. Also, if your dog has recently lost a good deal of weight and muscle mass, a backpack for doggy that is lighter and has a smaller overall capacity may be a better choice overall.