Donald N. Frey, the engineer who spearheaded the design and development of the Mustang, the spunky, stylish, affordably priced “pony car” that the Ford Motor Company rolled out in the mid-1960s in one of the most successful car introductions in automotive history, died March 5 in Evanston, Ill., where he lived. He was 86. /...../ Though much of the Mustang was borrowed from other Ford vehicles, including a Falcon chassis, the car developed an identity all its own for a younger generation in search of new looks and experiences. It was designed to appeal to both men and women, had...