Black Sheep was a mission for the GLA campaign that was cut before the release of the game, most likely because the mission consists of killing a bunch of innocent people with toxins. It was to be the third mission, and serve as an introduction to the toxin tractor.

In 2009, to celebrate the then-sixth anniversary of C&C Generals, EA released the mission for free download.

Originally posted by EA:

In celebration of the sixth anniversary of the award-winning real-time strategy classic Command & Conquer Generals, we are releasing an exclusive gift to Command & Conquer fans — the lost mission of Command & Conquer Generals. The never-before-seen and controversial Global Liberation Army mission was not included in the game’s original 2003 release and has been hidden and securely stored away in a secret location at our EA Los Angeles development studio until today. Command & Conquer, download the mission Black Sheep for free and unleash their inner General.

Black Sheep was originally slated to be the third mission in the Global Liberation Army campaign where players were given command of the Toxin Tractor, a slow-going farm vehicle modified to spray a deadly corrosive agent, and ordered to eliminate the town that had been corrupted beyond salvation by the USA’s propaganda. While the mission was ultimately removed from Generals, the Toxin Tractor was available in other campaign missions, as well as in multiplayer and skirmish modes.

Go on ModDB ( https://www.moddb.com/games/cc-generals/addons/generals-black-sheep-the-lost-mission ) and download the file.

Unzip it, and paste the folder to your Documents\Command and Conquer Generals Data\Maps folder.

You can access it in-game through Skirmish.

Have fun!