This is very common with dogs. It means that they have a vitamin deficiency. If you put a little salt in the cat's food, the dog will not like it. I had a dog that used to pick up her deposits and carry them into the house and drop them inside. This is what I was told to do, and it worked. But I have observed many dogs at the litter boxes doing such. Your dog is not uncommon. Best of luck to you.
