The film shows the human carnage, of course. But they somehow trained the horses to lay down, writhe, and scream. Yes, horses can scream. That sound was even more horrible than the gore.
Cavalry has a storied role in Polish military history, the most prominent example being the winged hussars. Horses were fitted with saddles that had large upright wings made of turkey feathers. These not only made for a striking appearance, but when the Polish winged hussars charged into battle, the rattling and whistling sound of the wings was said to disturb opposing cavalry and often put them into disorder or flight. Only careful training made Polish horses tolerate carrying the wings.