About a year ago, I was looking for somebody to do a mount run with in Zul'Gurub (back when I was a noob and couldn't solo it.) I got somebody who agreed to run it with me if I would also do Karazhan with him for the mount in there. We ran ZG and no mounts dropped (and sadly, they never did for me), so we went on to Karazhan, it being my first time in there. And, what do you know, the mount drops, and I win it. =D The other guy was furious, since he had apparently been running it for a while. I felt bad, so I did offer to run it with him whenever he wanted, but he never got back to me on that. Still, it is my ground mount of choice, and I am still very happy to have it, since I don't see many people on my server with it.
For a more recent one...I am currently working on the Salty title, and I was absolutely thrilled when I happened upon the Sea Turtle mount a few minutes ago. I didn't think that I would ever get it, since I heard that it's something like 1 in a couple thousand or something like that. I am very happy. =) Share your happiness with me! Tell me your stories!