Since the MUSTANG (pony) was the first, all the others caught on as a class of cars, like the compact, sub-compact and full size. Remember back then there were not that many sizes out there.

Some early Mustangs are muscle cars – most are not – but they’re all pony cars.
The basic design of a short deck and a long nose on a relatively compact vehicle started off the pony car craze – the Mustang, Camaro, Firebird, Barracuda, etc….etc……

Mustang=horse
Pony=horse

All Mustangs are called pony cars. While it’s true a real Mustang is a wild horse, the automotive Mustang is also adorned with horses, i.e. the front grille has a chrome, running horse or “pony”. Other horses can be found around the car as well, all leading people to call that particular Ford, the pony car.

Simple as that!

What Matt said.

In fact, the term is widely used not only for Ford Mustangs of other years, but also for direct competitors from Chevy and Dodge.

a MUSTANG is a wild HORSE. a PONY is a HORSE.