Weren't pony cars different from muscle cars?
The line between them is a bit fuzzy. IMO, pony cars are generally those vehicles that were the direct competitors to the Mustang. The Mustang was the original pony car and preceded the Camaro, Firebird, Challenger and Cuda which were produced to compete with the 'Stang.
The original muscle car was arguably the Pontiac GTO that DeLoren did when he at Pontiac. Most muscle cars are intermediate size cars (in the 60s) that had few luxury/comfort items (ie, a stripper) but had a monsterous motors, allways a V8. The Ford Torino, Chevy Chevelle SS, Dodge Charger, etc were made to compete with the GTO.
No pony nor muscle car ever had any other engine than a V8. There were not import cars.
Some of the vehicles called "muscle cars" today are not in the classical definition of the term.