Depends on how strict a person wants to be. Some say the 1949 Olds was the first Muscle Car. An argument could be made that it was a variety of even earlier cars.
Dodge Chargers and Road Runners are much larger cars than many of the smaller Muscle Cars.
It is argued whether Mustangs are Muscle Cars or not.
Some include Corvettes, some do not.
It's all debatable.