Mercury is a clothing brand in Grand Theft Auto: San Andreas.
Mercury hoodies are available from Sub Urban for $70, and they also sponsor Hotring Racers. There also would have been three Mercury T-shirts available to purchase, however, they were cut. Their textures can be found in the game files.
The company is based on the real-life clothing brand Quiksilver, due to the similarities of the logos and the fact that the element "mercury" is sometimes known as "quick silver".