On Windows, there used to be (possibly a third-party application) a desktop widget that had a "turtle", and if you clicked on the widget it would drop a little pixel of food, and the turtle would slowly walk over to it and consume it. I thought that was really cool.
Well we xroach and xbunny years ago. The former would have roaches scurry on your screen when you closed or iconified a window. The latter used X11's normally lax security and you would have your engergizer bunny across multiple people's desktops. Good times.