The Galpin Rocket showed up at an auto show as a carbon-bodied, supercharged reimagining of the Mustang co-designed by Henrik Fisker, then seemed to disappear before anyone could buy one. A Michigan low-volume manufacturer picked the project back up and put it in its own catalog. What emerged made 725 horsepower and carried a six-figure price. Read on for the story of the wildest Mustang that never came from Ford.