I think calling any of the GM's a "Pony Car" is just wrong. This terminology should only be used for a Ford Mustang. Mid-size Muscle Car would be a better designation.

I always thought that "pony car" was what they called the group of cars with a short wheel-base. Which is Camaro, Mustang, Trans Am..... these would all be in the "pony" class when racing together. and a MUSCLE CAR is any car with "BIG MUSCLE" under the hood.

It is referred to as a "pony" car simply because of it's size. The Camaro was built to compete with the Ford Mustang. Mustangs had an optional "pony" interior. I would assume the illiteration just kind of took off from there.