There is an issue on MoP classic with the new glad mount that just got sent out from pvp season 11. For some reason you can only use the Cataclysmic Gladiator’s Twilight Drake mount on the characters you learned the mount on, while any other glad mount from tbc / wotlk is account wide in the collections tab, you can use all those mounts on any char since the collections tab was added.
A gm ticket response told me he could see the mount learned in my collections tab, from my rogues (Dimagz & Dmagz on Benediction) although when I log my war (Dmags-Benediction) I don’t see the mount in my collection, only my 2 rogues that got the mount in the mail and right clicked to learn it have it in their collections tab.
Think this is a bug similar to what happened when prepatch came out and all the glad mounts among others, got deleted from collections lol