Nago was a giant boar. He was shot by Lady Eboshi with a gun and he turned into a demon. At the beginning of the movie, he is seen as a demon, looking like a giant bug with wormy tendrils growing out of him. He is shot in the eye with an arrow by Ashitaka, the last prince of the Emishi people. Nago is shot again, and the tendrils leave him. He falls over and the wise woman of the village says that they will make a mound for him. Nago says,"Disgusting creatures. Soon, you will suffer the pain I have suffered." Then he quickly decomposes.
He was also "BEAUTIFUL AND STRONG" as a boar that he was very close with said later in the movie, when confronting the wolves about them letting Nago die.
END OF SPOILER WARNING