I had a 2 pony cars in 69 a 67 Mustang and a 68 Firebird...
the "pony" expression I think its because its not a fullsize horse...its a smaller car than the Mid size Muscle cars...I agree w Mr P Body if thats what he`s saying...Muscvle cars were typicallymid size cars with big engines...when pony cars came out with the same size engines as Muscle cars(Firebird 400 and GTO for example)the pony cars blew the muscle cars away! So nothing to fret about....it was just tough for GTO salesmen in 69-70 to sell a muscle car that was slightly slower and more expensive than the Firebird!
check out Wikipedias explanation...and history behind the name...yes, Mustangs started the pony car platform... http://en.wikipedia.org/wiki/Pony_car