The Onassis family have been famous ever since Jackie Kennedy joined their ranks in the 1960s by way of marriage to Aristotle Onassis. They didn't need the fame to bring them money though; they own a huge shipping company. The number of children in the family has dwindled as the years have passed, leaving Athina Onassis, Aristotle's granddaughter, as the sole heir of the entire empire. Nobody can say for certain precisely how much she's inherited, although it's understood to be far in excess of one billion dollars.
The way at which the young Onassis arrived by her fortune wasn't a happy one. She was only ten when her mother passed away, causing her inheritance money to be held in trust until she was much older. It appears the shipping company will have to find different ownership in the future; this heiress is far more interested in show jumping.