The Ford Mustang II is a small, front-engined (V8), open "two-plus-two" concept car built by the Ford Motor Company in 1963. Although bearing the same name as the first generation production Mustang, the four-seater Mustang II which closely resembled the final production variant that would appear in 1964, was intended primarily for the auto show circuit. After debuting at the 1963 Watkins Glen Grand Prix, the Mustang II had a short lifespan as a show car before being relegated to the task of "test mule". The sole example still exists, albeit in storage at the Detroit Historical Museum.
Sources: Wikipedia, OldConceptCars.com