It was originally planned to be a demon.
This concept was later scrapped, and the character was then envisioned as a human sorcerer's apprentice. The reason for the change is unclear, but it may have been due to concerns about the potentially dark or disturbing imagery of a demon character.
While the demon concept didn't make it into the final film, it's interesting to think about how the story might have unfolded with a demon as the main character!