Originally a muscle car was defined as a midsize car with the largest v8 engine offerings of the day. The Mustang, Camaro and Challanger we not midsize car. So they were considered "pony cars". They were small 2 door cars that still had v8 engines in them.
The muscle cars back then were good for strait racing. Nothing else. They were nose heavy and handled poorly and had brakes that were not up to the job of slowing these large cars with the power they had. The pony cars were lighter and made decent power but handled much better than then their muscle car brethren. Now decades later we have small engines making big power numbers in the Mustang, Challenger and Camaro. Not only do these cars outrun the old muscle cars but they out handle and brake them as well. Well rounded cars like the old pony cars. But by the original definition the Mustang, Camaro and Challanger would be pony cars. The Charger would be considered a muscle car because it is a mid size car and has the largest v8 motor the company manufactures in it.
I think people think the term "pony car" isn't mescaline enough. So they prefer the term "muscle car". Bottom line who cares what people call one car or another. haha There is no real right or wrong answer because as times change the original definitions or formulas do not work in the current offerings. Especially since emissions and fuel economy forced manufactures to make smaller and more efficient motors. They address the power numbers by adding forced induction. Who is gonna cry over that? Really? lmao
I'd rather have a car that makes big power N/A then add FI to it if that's the route I want to go to be honest.