Development mule

A development mule (test mule, or simply mule) in the automotive industry is a testbed vehicle equipped with prototype components requiring evaluation. They are often camouflaged to deceive competitors and thwart a curious automotive press.
Application.
Mules are necessary because automakers must assess new aspects of vehicles for both strengths and weaknesses before production. Mules are drivable, often pre-production, vehicles, sometimes years away from realization and coming after a concept car that preceded the design of critical mechanical components.
Mules may also have advanced chassis and powertrain designs from a prospective vehicle that need testing, which can be effectively concealed in the body and interior of a similarly sized production model.
If no comparable vehicle is available in-house or an external benchmark is being used mules may be based on another manufacturer's model. For example, in the 1970s the new powertrain package of first-generation Ford Fiesta was developed using mules based on the then class-leading Fiat 127, as Ford had no comparable compact model of similar size to utilize.
Mules are also used to conceal styling changes and visible telltales of performance alterations in near-production vehicles, receiving varying degrees of camouflage to deceive rival makers and thwart a curious automotive press. Such alterations can span from distracting shrinkwrap designs, somewhat reminiscent of dazzle camouflage, to substituting crude cylindric shapes for taillights, non-standard wheels, or assemblages of plastic and tape to hide a vehicle's shape and design elements.
Development mules are often used very heavily during testing and scrapped. Occasionally they are acquired by members of the automaker's engineering team or executives overseeing the design process.