- I earned the Nether Drake in Burning Crusade but I cannot use it on my characters
- I got a Frost Wyrm in Wrath of the Lich King but I don't see it in the mount collection
- I remember I had the Gladiator't Twilight Drake from Cataclysm but it's missing
Until Shadowlands, many PVP mounts were not account-wide and only the character that earned the FOS could summon the mount. Since Shadowlands, all PVP mounts are account-wide.
If you earned a Gladiator mount previous to Shadowlands launch, log in to the character that earned the mount to make the mount available to all your characters on the same faction.
Since mounts are now account-wide, if you transfer your character to a different account, mounts you earned with the character will not transfer, will remain on the original account. This is intended.