Tyrael's Charger by far is the rarest
You registered on the forums to post on this 1 year old thread?
The rarest flying mounts in the game are Big Love Rocket, Infinite Timereaver, Solar Spirehawk, Heavenly Onyx Cloud Serpent and Thundering Cobalt Cloud Serpent.
Shit that isn't available to actually be acquired in the game doesn't count. Especially stuff that was incredibly easy to get and was removed after.
My horse was flying when i was in netherstorm in outland... I'd summoned the wrong mount as i jumped off the edge... Does that count as flying or falling with grace?