The wolf is the shepherd's worst enemy. He sneaks in under the cover of darkness and swipes some sheep for dinner. If and when a shepherd manages to catch a wolf he will beat him mercilessly taking out on him all his frustrations for all the sheep that have been lost to wolves in the past. Did this wolf necessarily steal all those sheep? Probably not but the shepherd doesn't really care. To him this is payback for everything.
The Knaf Renanim adds that the reason why the Torah says that a thief who is caught pays double is because it is probably not his first time stealing. Therefore we make him pay back for the "other time" as well.
The lesson we learn from the wolf is that if you steal or cheat you may get away with it a few times. But if and when you do eventually get caught you will pay not only for what you were caught doing now but for all those who stole before you as well as for your own previous crimes. You will be the whipping boy who will pay the price.
The Wolf says, "Al Kol Dvar Pesha" we are punished not only for our own crimes but for all other crimes that our victim suffered from in the past. "Yishaleim Shnayim LiRei'ayhu", we will pay double because we already got away with it once.