The show, filmed at Santa Anita, starred Dustin Hoffman but it was canceled in March 2012 following the deaths of horses. The suit claims four horses died during the show's production, not three as previously reported.
Barbara Casey, an American Humane Association employee who oversaw animal activity on the show, says she witnessed numerous instances of mistreatment. Horses were "often drugged to perform," and "underweight and sick horses unsuited for work were routinely used" by producers, her lawsuit alleges.
Casey claims that when she tried to complain about the conditions to her bosses, they fired her. She is also suing the humane association for wrongful firing from her post, which she held for about 13 years.
The Humane Association is not commenting on the lawsuit. HBO insists it took every precaution to ensure horse safety.
Casey served as director of the association's Film and Television Unit, which oversees animal welfare and often allows a notice to be attached to the end of films and television shows that says no animals were harmed during production. The nonprofit association's film- and TV-monitoring efforts are paid for through entertainment industry grants, according to the lawsuit.
The Associated Press contributed to this report.