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The Big Metal Unit is an Easter Egg that you can get forged when you have the Gold Pantaloons from BG1, the Silver Pantaloons from BG2 and the Bronze Pantalettes from ToB. You'll need to have imported a character from BG1 obviously, otherwise the Gold Pantaloons will not appear in the game. Once you have all three you can have it forged by a mystery gnome smith to get the following:
The Big Metal Unit