Design and personality
In her earliest appearances, she was a very conservative and innocent rabbit, very closely resembling the character designs of Ortensia and later Minnie Mouse. Toward the end of her run in her Oswald cartoons, she became progressively more sultry and flirtatious, culminating in her last appearance in All Wet, where she ended up bordering on femme fatale.
Naming and usage
Like Ortensia, her name was never clarified to the public in the shorts, only seen on official copyright documents and the book: Walt in Wonderland: The Silent Films of Walt Disney. Curiously, she was identified as Ortensia herself back when Ortensia was nameless (despite being a different species) in Universal Pictures' official copyright synopsis.
Fanny was originally supposed to be in Epic Mickey, but Ortensia ended up being used in her place.
Fanny under Walter Lantz
Fanny reappears in a 1934 Lantz cartoon, "Candy House", and was reworked to be Oswald's sister instead of his love interest.