I’m puzzled by the myriad of references to various figures like Octavian (Octavius? Octavianus???) Aurelian or Domitian, who seem to possess Latin names distinct in structure from the names to which they are similar.
The “-ian” suffix that you see in a lot of these names indicates adoption into another family. The original nomen or cognomen would have “-anus” or “-inus” added and then be appended as an additional name to the adoptee’s new name. The classic example is the adoption of Gaius Octavius by Julius Caesar; he became Gaius Julius Caesar Octavianus, or Octavian (to distinguish him from his adoptive father. (He then, of course, later became Augustus, or (to use the full official name, Imperator Caesar divii filius Augustus.)