When my seven brothers and sisters and I were growing up in Valdosta, Ga., in the early 1970s, we had a pony that nobody ever rode, because it was too small and it was mean. One day in 1973, our father, Gilbert Speed, traded the pony for a 1965 Ford Mustang. He traded a pony for a “pony car,” as Mustangs came to be known. That is how it all got started.

Our...