Boss cars
Some race events are listed as "Boss Race" in the event description. Win that event under the gold difficulty setting to be awarded that Boss's car (1969 Dodge Charger, Subaru Impreza WRX STi, Chevrolet Corvette Z06 and Nissan Skyline R34 GTR). When you win a Boss car, it will appear in the Pocket Garage, and you can race it in any game event.

Boss car warning
When considering buying a car, do not buy any of the cars that are used as Boss cars. If you buy one, you will end up having two of the same car. Your garage can only hold 20 cars, and there are 24 cars total. The four Boss cars have their own separate garage and do not take up any of the twenty slots. You cannot get 100% completion if you buy any of the cars used as Boss cars.