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The Tauren Marine was first seen as an April Fool's Day prank conducted by Blizzard in 2008. The website for StarCraft II was updated with an announcement that the Tauren Marines were the newest Terran Unit.
According to the unit description page, StarCraft's Terran Confederacy discovered an obscure planet called Azeroth, which was home to "a dizzying selection of humans, mutants, and aliens in a state of perpetual superstition and conflict." The Confederacy decided that the Tauren, one of the most physically imposing of Azeroth's species, would make excellent additions to their space marine corps., and equipped the large number of young bulls who were interested in their proposition with state-of-the-art armor and weaponry.
In StarCraft II, if you head to the north end of the map the "Zero Hour" mission, past the Zerg Base in the northeast section, you will see a Tauren Space Marine near an outhouse. His name is LV80TSM! and upon getting close to him or clicking on him, he rushes into an outhouse. The outhouse cannot be destroyed.
A Tauren Marine unit model in StarCraft II