The Boar Demon is a character in the film Princess Mononoke. It was shot with an iron ball causing it to be wounded and turn into a demon. The demon became fueled by hate and attacked Ashitaka's home village. Ashitaka then fought the boar and defeated it, but he became infected due to a wound from the boar. The wound causes Ashitaka to do very violent things and to be healed of it, he had to leave his village and flee to the west.
Later on in the film, more boars are met and there becomes an argument over the Boar Demon, as they believe that Ashitaka was evil for killing it, they also disbelieve that the boar was actually a demon.