Baron Rivendare's mount used to arguably be the rarest drop in the game:
Just to get an idea of how rare this is.. (taken from the WoWhead comment)
As of 2/2/2008, Wowhead has only seen 1 drop out of 5290 kills...
(1/5290) * 100% = 0.019%
For one run of a given instance, the odds are higher that...
Ashes of Al'ar drops (90x more likely)
Swift Razzashi Raptor drops (63x more likely)
Swift Zulian Tiger drops (31x more likely)
Baron's Runeblade drops (9x more likely)
You are audited by the IRS
You die from slipping in a bathtub
You have a life
Personally I've been playing on and off since release and have only seen it twice across two servers. I've seen the black bug mount (the reward for a lengthy quest line involving 40 man raids at the time) more than that.
Well recently they changed it to be 1 out of 100 chances. Since then I know of 2 people that have gotten it to drop. Now people are farming it like they'd farm herbs. I've tried a few times, don't know if I have it in me to keep at it. I believe it's an achievement as well so that will make it more popular.
I can see now whatever they make an achievement they tend to want to make it more possible that people can nibble the carrot. If you keep it on a pole on a string too far away players just get angry.
Wartales - A rough, low-fantasy gem
23 hours ago