An important treatise on medieval music theory was written in the 13th century. His or her name was lost to history, and a 19th-century French historian later dubbed him/her "Anonymous IV". This styling implies that there were at least three other Anonymouses (Anonymi?) discussed in the same work, but I have been unable to find any information about them. Who were they, and what contributions did they make to music theory & history?
There were others. Some one (or some authors) called the authors of several treatises "Anonymous #" for various numbers (#).
https://en.wikipedia.org/wiki/List_of_music_theorists lists quite a few and assigns each a work, that one wrote. THere are also some not having a number but a location instead.
Anonymous 4 seems to be the most commonly referenced.