A mule is a test vehicle which uses the bodywork from an existing production car with new mechanical components. Mules are most commonly used for development of the powertrain and suspension.
Often the bodywork is modified to suit the new mechanical components and to alter the mule's centre of gravity, overall weight and weight distribution. These changes are often made quite crudely, so mules often have a shabby appearance.
In some cases mules are built with some production-spec bodywork. For instance, the nose of the new model might be grafted onto a body largely composed of old-generation panels. This might be essential for developing engine cooling, braking and HVAC systems.
Mules are often painted black and/or white, and may carry disguise panels so that it is more difficult to discern the make of car – in fact some manufacturers have been known to deliberately incorporate styling cues from rival vehicles in their disquise, such as BMW 'twin kidney' grilles.