not something that is serious! this is pretty common in dogs. it's probably one of the following things. the first is worms. the anus itches as the worm move behind it. this can cause the pup to slide with it's butt on the carpet. or it could be that his anal glands are clogged. this causes the same symptoms. for the first you need a good worm tablet and for the other you need the vet to pinch the contents of the glands so that they are empty. this could be a reaccuring little problem and the vets may teach you how to empty them if it reoccurs!