It was 4am one night, I was dead tired, with 2 friends. We decided to do Karazhan for nostalgia (it was during ICC WotLK), and I ported to Stonard and we started heading there. One friend fell asleep mid way, and I was getting there and just wanted to do it tomorrow, however friend was like "Nah lets at least go up to Opera Event.
We zone in, clear to first boss, kill it and the Horse drops. I had never seen a rare mount drop up till that point, and he was just like "Hey, I don't really want it, go ahead and take it." So I won it.
That is how I acquired my Fire Unicorn Vampire Horse.