Sonic.exe

Sonic.exe takes form of Sonic the hedgehog, a popular video game character, but is truly a terrifying virus entity. There is only one way to make him appear and that is to find the Sonic.exe game disk on level 4: Abandoned Office. This level is the only place that the disk is found, unless someone has already found it. 

Once you have found the disk, keep it until you find a working computer, then insert it into the computer. The original sonic game will start up, showing the title screen where sonic is staring back at you, making the atmosphere very unsettling. However, if you start the game, and lose, you will be teleported to an empty, gray room and a screen will show up saying there was an error.

Sonic.exe will then climb out of the computer and proceed to try and talk to you. He is one of the smartest entities because he is able to have a full conversation with you. Though he will just ask you to race him and even though he asks you don't really have a choice. you will be teleported to level: Green Hill Zone where there is a mile of fake grass with a red sky. The race is rigged against you for Sonic.exe to win. Once you lose, you will be teleported back to the gray room where sonic will be facing away from you looking down. He will constantly repeat "You lost. You lost. You lost," until finally he turns around and his face has changed. He will chase you down until he catches you.