General: original
Usually featuring an original character (and setting); that is to say, one which is not presented in a copyrighted work created by another artist or corporation.
This tag appears to be an artifact of crossposting between sites such as danbooru, and it's validity on e621 is questioned.
As an alternative method of finding 'original' content, one can search for posts with no character tags, or copyright tags (chartags:0 copytags:0). Though the efficiency of this method depends on a number of factors, some of which assume that every post has its character and copyrights correctly identified.
