It's "Dogs" by Pink Floyd from their 1977 album Animals. The album is a retelling of sorts of George Orwell's novel Animal Farm in which pigs were the dominant species on an English farm, dogs were their police force and sheep were the common citizens and unwitting slaves to the pigs and dogs.
The song itself is just over 17 minutes long. When the episode entered syndication (reruns), the song was removed and replaced because of licensing issues.