One of the rares mounts ingame, but after MOP it became super easy to solo Kael'Thas for most lvl 90 classes. So atleast you wont have to roll against 10++ other players when it drops!
The drop rate is around 1-2%. As in, I got it on my first solokill, and second kill ever. One of my friends have killed it 120 times and counting, but have never seen it. It's in other words totally random.