English sportscars invade the U.S. by the boatload, the Italians are always glamorous, and even the Japanese get into the 2-seater game with the immensely popular Z car.   Midway through the decade, Lee Iacocca introduces the most successful new car ever, the Ford Mustang.  It's followed by the Chevy Camaro, Pontiac Firebird, Plymouth Barracuda, Dodge Challenger, American Motors AMX, FoMoCo's own Mercury Cougar (did we leave anyone out?) and they're all called 'ponycars', in honor of the original.  Then there are the 'personal cars', the 4-seat Thunderbird, Pontiac Grand Prix, Oldsmobile Toronado, Buick Riviera ....  And finally there's the horsepower race and the re-creation of the 'Musclecar', the idea of putting a big engine in a small chassis to make a vehicle go like stink.  While some pundits credit the Pontiac GTO as the first of the genre, it ain't.  There was the Studebaker Golden Hawk in the '50s and the Stutz Bearcat long before that, in the nineteen-teens!   But 'sexy' is the word for the period, as evidenced by the car that Enzo Ferrari called the 'most beautiful car ever created' ... the Jaguar E-Type, aka XKE.