- I have the Nether Drake mount but no Feat of Strength
- I have the Gladiator title from Burning Crusade but I don't have an achievement
- I am missing achievements from season 1-4
The achievement system was introduced in the Wrath of the Lich King. For this reason, there is no Achievement for seasons 1 to 4.
If during the Burning Crusade, your character ended at the top 0.5 of the ladder for your Battle-Group, you would have earned a title (Gladiator, Merciless Gladiator, Vengeful Gladiator, Brutal Gladiator) and a Nether Drake mount (Swift, Merciless, Vengeful, Brutal). Both the mount and the title are character specific, not account-wide.
If your character has the title and mount, your Achievements interface should also show a Feat of Strength (not an Achievement).