A muscle car by definition is a standard intermediate sized two door sedan with an optional performance package. Most credit the Goat with that distinction but I beg to differ.
Me, too. The first muscle cars would be the '62 Super Stock Dodges and Plymouths, IMO. That poses a problem for the definition. The 1962 Dart 440/330 and the Fury/Belvedere/Savoy were positioned in the sales wars as full-sized cars, regular cross-shopped with the Impala and Galaxie. Were they full-sized or not? True Dodge trotted out the 880 later in the year as a full-er sized car.
Secondly, how does this definition address the 340/383/440 A-bodies, the Firebird 400s, the 302/396/427 Camaros, the FE & Boss engined Mustangs/Cougars, and the 390/401 Javelins. Are these not all muscle cars? What about the R/T Dodges, GTX, and Fairlane/Comet GT? These were hardtop and/or convertible only.