The 1969 Mystery Mustang was an experimental concept car created to revitalize the Mustang brand. This "mystery" Mustang doesn't look like any 1969 Mustang ever offered to the public. In fact, it has more the look of a De Tomaso Mangusta or some other exotic import car from Italy.
During 1969, Ford Motor Company held a secret styling campaign to create a new look for the future (that is, for 1974: the year it hoped to recharge Mustang's image). Some had complained that Mustang was getting too obese, outgrowing its initial youthful appeal. So in 1969, a number of experimental Mustangs were shaped up by Ford's styling department. Some were made of clay and wood, and had no engine. A few were complete, driveable cars.
It wasn't revealed whether the car you see here had an engine, but it was the best-looking of the various dream cars projected. It was larger than the severely downsized Mustang II, which was eventually chosen for 1974. Moreover, the car seen here did not have the "Mustang look" and was voted down.
From time to time, a few "dream cars" are preserved. Most are scrapped and virtually forgotten. Some years ago, a junkyard of discarded experimental Studebakers was discovered. But no group of car collectors is known to have come to the rescue to restore and preserve the abandoned hulks. Most other auto manufacturers didn't even have a junkyard in which to toss their abandoned projects, but just scattered the pieces to the wind, so to speak.
As mentioned before, a few dream cars still exist in car museums or in private collections, despite the manufacturer's preference for secrecy regarding new designs.