There are so many great candidates, and one of them is in the header image. Epicus Maximus briefly appeared on the Cataclysm beta, before being added to the Brawler's Guild as a late-rank boss. Actually, there are several Brawler's Guild bosses which would be excellent as mounts. At the exact opposite end of the scale, the Harvester Golems in Westfall are something I've always wanted to have as a mount. We've got something similar in patch 5.4 with the Sky Golem, but it's not quite the same as those adorable little harvesters.
And I haven't even begun to start thinking about raid bosses, dungeon bosses, any of that. So what would be your primary target? Dream big! Anything's possible!