The initial reason the job-specific mounts were rewarded is to encourage people to play those classes. PLD and WAR received their job-specific mounts only after completing a certain number of 'difficult' duties as that class (aka, not just hard modes, but extremes or coils...). WHM (technically CNJ) did not have a defined reason for receiving the unicorn, and to be honest... the quest was kind of lackluster. It may have been to encourage people to level healer, but possibly no. I imagine that other jobs may received job-specific mounts in the future, but it won't be a priority because there's no functional reason to 'entice' people to play dps classes like there was to entice people to play tanks and healers.