What information on this site led you to believe that the story of your aunt being named after a racehorse was not true?
There was indeed a race horse called Dorigen who won a race called the
William Hill
Lincoln in 1933:
http://www.racingbetter.co.uk/race/doncaster/lincoln.html
Your grandparents may have had the year a little off, but just because the name Dorigen wasn't invented for the horse, it doesn't follow that your aunt wasn't named for it. :)