Unlike a "Muscle" car, the Mustang, or any other pony car from that era could actually be made to go around corners... Of course, the ability of going around coners isn't really high on the list of classic car "aficionados" of today.
You can always look up the definition of muscle car and pony car on the web, wikipedia being one source. The terminologies are rather disputable, but give a realtively good idea. As far as the "pony" nomenclature is concerned, you do know what a "rat", "mouse", "elephant" and "Indian" are, right? The "pony" is similar, in that it's just a name.