Animal Aid said it was told that a four-year-old filly had died after the final race on the second day of the 2010 Aintree Grand National Meeting.
"After carrying out further investigations, we discovered that the young filly, known as Baba O'Curragh, had suffered a knee injury and was destroyed at the racecourse," the group said.
Her death meant that five, rather than the reported four, horses died at the three-day meeting in April. The other fatalities were Prudent Honour, Plaisir D'Estruval, Schindlers Hunt and Pagan Starprincess.
Five horses died at the 2009 event, and a total of 35 have perished since 1999.
Animal Aid's horse-racing consultant, Dene Stansall, said the inability of authorities tied to racing to tackle the death rate on British racecourses "highlights a serious and deep-seated problem".