Original fantasy and derivative doujin were two different concepts. The specific differences were as follows: 1. Original fantasy refers to the original storyline and character settings in novels, anime, games, and other works created and determined by the original author. Derivative doujinshi was based on the original fantasy works created by fans or creators, including storylines, character settings, illustrations, music, and other aspects of creation. 2. Original fantasy works usually have a deep cultural background and historical value. Derivative Doujinshi works are the second creation of the original fantasy works. They can be closer to the spiritual core of the original work and add new elements and charm to the original work. 3. The difference between original fantasy and derivative doujin lies in the purpose and content of the creation. Original fantasy focuses more on the protection and inheritance of the original work while derivative doujin focuses more on the creativity and uniqueness of the creation itself. 4. There is also a certain connection and interaction between the original fantasy and the derivative doujin because some original fantasy works will also attract the attention and creation of the derivative doujin creators.