A lime-green Cougar so radically reworked that showgoers assumed it was a new model altogether — that was Mercury’s 1970 El Gato concept, unveiled at back-to-back auto shows before vanishing for good. It borrowed its fastback roofline from a Mustang, swapped its back seat for a European-style cargo shelf, and tested run-flat tire technology that failed spectacularly. Ford’s standard practice of destroying show cars means no one has documented seeing it since 1970. Here’s the story behind Mercury’s strangest concept car.