I think you should reward Deadly Gladiator's Frost Wyrm to all Gladiators on Frostmourne realm, like it was back then on retail.
"Arena mounts are epic fast flying mounts reward awarded to top ranking arena teams at the end of an Arena Season. Teams earning these mounts also earn a corresponding Gladiator title." It would make more sense considering how this realm work.
If you're not going to do it, would you please explain why?
That was a reward for Season 5 on retail, while we are on Season 17. The fact Frostmourne is on progressive content release doesn't change that and I don't see why it should get special treatment on a reward all WotLK servers might want, but not have a chance at getting like that.