Tomorrow is pre-patch day and all Shadowlands raids will be permanently Fated, which means you can get the Jigglesworth Sr. mount in just one week instead or 3!
You’ll receive the mount by completing all 3 Shadowlands raids on Fated difficulty. The meta-achievement (Fates of the Shadowlands Raids) is going away when Dragonflight launches. However, you can now complete it in a single week in the Dragonflight pre-patch.
You can work on the Fates of the Shadowlands Raids achievement on multiple characters/alts, but you must earn the partial achievements (e.g. Fate of Nathria) by clear the whole raid on 1 character.