The horse had to be put down on Tuesday morning (13Mar12) after it reared up, fell backwards and broke its neck in the stables area of the Santa Anita set in California.
The tragedy marks the third time a horse has had to be euthanized - two creatures were put down last year (11) during filming on the first season of the show, which stars Dustin Hoffman.
Following Tuesday's incident, People for the Ethical Treatment of Animals (PETA) officials called on producers to cease filming. A PETA spokesman said, "HBO should be ashamed. Three horses have now died, and all the evidence PETA U.S. has gathered points to sloppy oversight, the use of unfit or injured horses, and disregard for the treatment of racehorses."
But HBO bosses insist that's not the case - and they are doing all they can to minimalise on-set injuries.
In a statement sent to WENN, a spokesperson says, "From the very outset of this project, the safety of the animals was of paramount concern to us. Recent assertions of lax attitudes or negligence could not be further from the truth.
"We partnered early on with the American Humane Association, which is the only mandated authority in the industry, and we work very closely with the AHA and racing industry experts to implement safety protocols that go above and beyond typical film and TV industry standards and practices.
"For example, pre-race exams are performed by a highly experienced California Horse Racing Board official regulatory veterinarian before each race scene. Everyone associated with Luck cares deeply about the well-being of the horses who are so much a part of the heart and soul of the production."
But producers have agreed to halt all horse-related filming until AHA officials complete their inquiry.
The spokesperson says, "Production will continue with other scenes."