So followers from what I can tell
The Original 3
Blood - Koltira Deathweaver
Frost - Thassarian
Unholy - Highlord Darion Mograine
The Four Horseman
Nazgrim
Thoras Trollbane
?
Tiron Fordring
Others
Amal'thazad (The Frost DK Trainer)
Initiate Whitemane (she better keep her hat)
Rottgut -Monstrosity probably, whatever Rotface and Festergut are