The Steyr Aug was a beta weapon in Grand Theft Auto: Vice City. It appears to be an assault rifle, which means that it would have taken the assault rifle slot. Its model (steyr.dff) and its textures (steyr.txd), still remain on the PS2 disc. These files were removed from all other ports of the game.
The weapon is based on the weapon of the same name.
In Grand Theft Auto: San Andreas, a Steyr AUG can be seen among weapons on the shelves/racks inside Ammu-Nation.