The original Mustang concept was a completely different kind of car.
It is a little-known fact (except, perhaps, among gearheads), that the Ford Mustang was originally meant to be a far more exotic vehicle. The roofless, two-seat, mid-engine and four-cylinder powered roadster made it as far as the concept phase before someone at Ford remembered that the original Thunderbird, also a roadster, hadn't sold all that well. The concept was scrapped, and instead, Ford debuted a massively successful car which also created the pony car niche.
The only element of the original car which carried over to the production vehicle were the air intakes, located behind the doors, which were transformed into nonfunctional styling cues. In fact, the concept probably has more stylistically in common with the original Corvette Grand Sport than it does with the original production Mustang. Ford recently released some old period photos of the original concept which are really quite interesting. Though the actual car was an undeniable success, it would have been interesting to have gotten this car as well.