In general, "Anonymus" designates any piece whose composer is not known. For example, certain early music (often, pre-baroque) and folk music works are termed Anonymus.
Anonymus (Canadian Metal Band):
After the release of their critically acclaimed top 10 albums, Anonymus have enjoyed nationwide success. The band has sold no less than 30,000 albums, appeared in many videos and in several movie and TV soundtracks. Their career has been marked by a never ending journey of pure sweat and determination.