Blizzard in a Q&A on achievements was asked about bringing back old mounts. This was an interesting question because it challenges a Zul’Aman philosophy Blizzard had created.
Basically before Zul’Aman the reward for doing really well was gear. However, potentially expecting easier content pushes from people there needed to be a new symbol of doing well. You can get gear from Zul’Aman but only the best can get a Amani War Bear. This mount was earned by beating the first four bosses of Zul’Aman extremely fast. For every boss you cleared fast you would get rewarded bonus loot. For example if you beat three bosses quickly you would get rewarded a nifty ring.
The rings eventually were not seen as the big reward as people started disenchanting all purples and just picking up mounts. Blizzard knew that the value of these mounts would last far longer than the value of the epics. Even legendaries were replaced quickly. I can remember the debate of whether it’s better to use Thunderfury or a tanking weapon with superior stats.
Naxxramas tier of Wrath of the Lich King was the first test for this theory, which ended up being very true. Everyone and there mother was doing all the stupid achievements Blizzard laid out so they could get Glory of the Raider. Blizzard said that when the next tier of content ends this proto-drake would no longer be available since gear would make these achievements a joke.
I joked to a friend that Blizzard is so unoriginal they might call the next tier of content “Glory of the Ulduar Raider.” So the patch comes out and Glory of the Ulduar Raider is out. As expected the drake requirement is removed from Naxxramas.
So let’s fast forward to Trial of the Champion. As it ends up so few people have completed Glory of the Ulduar raider that blizzard felt it was necessary to keep the reward there. The same became true of the next two drakes. That is, as soon as blizzard started designing hard content that would always remain hard the drakes stayed there. When Cataclysm launched these drakes simply did not matter.
The question was almost specific to the Naxxramas achievement mount but there are many mounts that are not in the game anymore (that people still own).
Included in these mounts are:
Black Qiraj Mounts: There is the black mount rewarded only to a single person on each realm during the AQ opening event (which no longer exists).
Amani War Bear: Previously stated rewarded for completing Zul’Aman very quickly.
Season 1-8 Gladiator Mounts: At the end of every single season a new mount is rewarded to every gladiator. Easy fix is if you get gladiator you get access to all mounts from every season previously.
Glory of the Raider: Rewarded for completing all of the Tier 1 Wrath of the Lich King raid achievements.
I will say that the Glory of the Raider mount SHOULD be re-instated. In truth the mount was not all that hard to get. I can say on my account the only achievement I didn’t do was the 20-man Malygos for the 25-man version of the achievement and Speed Malygos for the 10-man version. Other mounts can be worked back in the game in some way to make it still a challenge to get them.
Edit: Apparently the three colored AQ mounts still exist. Walked in and the tenth mob I killed dropped one.