Zazie, nowadays you aren't going to find many people who will openly brag about making a fortune betting on horses for the simple reason that a lot of the people who bet large sums of money are doing it "offshore," through sites that don't report winnings to the IRS or other tax authorities. People who are making a ton of money doing this sure aren't going to advertise their winnings in a way that could bring the tax man to their door.
If you want to go historical, there are people who made a lot of money betting on horses and gambling in general. One of the more interesting people is Leonard Jerome, one of the men who founded the Jockey Club, the governing entity for Thoroughbred racing in the US.
Leonard Jerome is particularly interesting because he was also involved in politics. There is a lot of information available about him; these are just a few websites:
http://www.absoluteastronomy.com/topics/…
http://www.spiritus-temporis.com/leonard…
http://www.statemaster.com/encyclopedia/…
Leonard Jerome would be a good subject for another reason: while very few people nowadays know who he was, his grandson is known to just about everyone: Sir Winston Churchill, the Prime Minister of England during World War II and one of the most important politicians of the 20th century, was the son of Leonard Jerome's daughter Jennie Jerome. (During Sir Winston's political career, his enemies would jeeringly refer to his mother's father as "an American horse coper [dealer]." This would be like referring to Henry Ford as a car salesman: basically true, but there's a lot more to the story than that!)
Here in the US, the Jerome Stakes named for Leonard Jerome is still run at Belmont Park.