User blog comment:NaturallyTwisted/NaturallyTwisted's Guide to Hollow Evolution/@comment-24744083-20160409133830/@comment-7732249-20160413005949

The mask thing was information I got from my friend, I've never seen it (as I'm not that far into the Anime, but he stated that Arrancars are Adjuchas that have removed their own masks. The wiki stated that Menos Grande is typically an amalgam of many Gillians, and that some Gillians/Menos have slightly differing masks depending on the dominant Hollows within. And Gillians are a form of Menos, as Vasto Lordes are a class of Adjucha.