300 horses were used in the trilogy. Because most of the  scenes involving horses are intense battle scenes where the horses  could likely be harmed, a horse and rider were fitted with the same  type of suit that Andy Serkis (qv) wore for his role, and were  filmed in the studio doing typical "battle" things, like galloping  and rearing up, so the footage could be inserted digitally into the  battle scenes. In that way, no horses were hurt.