The classic definition of a muscle car is derived from a regular production mid-size car. The vehicle was upgraded with biggest V8 that would fit in the engine compartment, along with a better transmission, suspension, tires and brakes. Remember, mid-size car here refers to 1960s standards. A good example is Pontiac GTO, it was built from the Pontiac Tempest which was Pontiacs base model with optional small block V8s. The first GTO was a Tempest with an early 389cid, with other small upgrades, it was only available as a 2-door coupe.
The Ford Mustang, Chevrolet Camaro, Pontiac Firebird and cars like them were dedicated production coupes with better handling and performance. These cars were classified as pony cars. The two concepts were eventually merged, producing cars like the Dodge Charger, Challenger, Plymouth Barracuda and 70s era Mustangs. A few true muscle cars made it into the 70s, but died out within the decade. Cars like the Mustang and Camaro carried on through to the present and kept the muscle car name and legacy.