The old Windows 16-bit "Skifree" game (Ski.exe) from the original Microsoft Entertainment Pack, which used to be bundled with some Microsoft Windows 3.1x computers, has an easter egg:
Play the game, ski down the hill. Go past the finish line and keep on skiing. At some point, an ugly gray monster will jump out onto the hill and at the skier. After that the monster starts jumping up and down, rubbing his belly. The monster will disappear when you click your mouse on the game screen and you will be automatically teleported back to the starting line, where you can begin a new game.