Wow, what an ancient thread. Okay, I'll play, too.
It's a GT. Comparable to my Porsche 928, but not quite as sporty or as nimble, despite being close to the same weight. Not what is referred to as a muscle car in traditional parlance.
The GTO was the first muscle car. Intermediate coupe, stuff a big engine into it with a set of gears to do 0 to 60 and the 1/4 mile well. No handling necessary.
The first ponycar? If you go by what they are, that would be the '60 Ford Falcon coupe, which is the platform that the first Mustang was based upon.
When it comes to what is a muscle car, you could very simply say it's a lightweight, stripped down version of an intermediate, as defined by the GTO. However, as the late 60's rolled around, and all four American car companies started dropping in their biggest engines into the pony cars (which were smaller and lighter than the intermediates, perhaps thus making them even more 'muscular' than the musclecars, the defining characteristic started to blur, into what we have today. Is today's Mustang a 'musclecar'? Nope. But is the Shelby 'Super snake' a musclecar? You bet it is.
Or you can just call them all Hot Rods, because whether they are fast from the factory, or fast from made at home, it's all about speed. Got that from mom: 'If it's so much faster than it needs to be, it's a hot rod' which is what she called my '66 Mustang GT when I went and put headers, a Holley on it, and wider wheels & tires. OTOH, when I gave her a ride in my 928, she didn't even know when we had topped 100mph. Mom didn't survive to see my Mark VIII, having passed away at 90 y/o, in her sleep. But she is the one who said to go ahead and buy that first Mustang, when my dad wanted me to get some boring slow vehicle, like a 6 cylinder full size sedan (dad had a long history of hot rods when young, so I guess he was just trying to keep me out of trouble; didn't work, though!).