I don't know if it was just within my group of buddies in college and after or a general term that others used, but I remember back in the day (very early '70's) we referred to high performance cars as super cars. I don't remember muscle cars used as a reference until maybe 1980's or '90's. That being said I have always considered the 1964 GTO as the first super or muscle car; a mid-size car with a medium or big block high performance engine, stripped with very few if any options and no luxury features. The 1968 Roadrunner would be a good example of a true muscle car. Corvettes have always been sports cars. For me the hardest distinction is for the Mustang, Camaro, Firebird, Cougar, Challenger, Cuda, Javelin cars. In one way I would group them all as pony cars because of their size and layout, long front deck and short rear. But a car like a Boss 429, COPO Camaro, 440 6 pack Challenger, or hemi Cuda seems to fit under muscle car rather than pony car. Maybe it all depends on the size of the engine in it. Then there is the AMX. Sports car or pony car or muscle car? We could go on and on.