As a dog parent, who wants the best for your dog, let us assure you that you can fill the Kong with a lot of edibles. Peanut butter is one of them. If you are worried that it can make the Kong messy, you can always mix the peanut butter with some other munchies that your dog is a big fan of.  Besides, although it is true that it is difficult to clean a Kong which has dried up peanut butter in it, you can always soak the Kong in warm water for a while, which will make the process of cleaning the Kong much easier, thus managing the mess that your pet might have created.