I am experiencing a similar issue. I completed the unicorn quest as a Lv 30 Conj and received the unicorn whistle. I then switched to a Lv 30 White Mage and blew the unicorn whistle, which disappeared from my inventory. The unicorn did not appear but the "Unicorn" option appeared under "Mounts". I switched back to a Lv 30 Conj and tried to select the unicorn mount, but received the error "Unauthorized to summon mount". I have logged out and in several times but still could not summon the unicorn. I am now a Lv 32 Conj and still have never been able to summon my unicorn mount -- I always receive the same error that I am unauthorized to summon it. Perhaps this bug was caused by blowing the whistle as a White Mage, but that's just a guess. Has anybody else experienced an issue summoning their unicorn?