Anytime you get a new dog, you should take them to the vet right away for a check up. I would do that right away and have the vet check his poor butt! Shelters don't usually take the dogs to the vet. My dogs don't wipe their butts on the ground unless they have issues with their anal glands Maybe he was wiping his butt on the ground and has an infection. Let us know what your vet says.