Fathom Dweller

Deep within the hungering chasms of the Great Sea, swarms of Fathom Dwellers sway like a great floating carpet, sightlessly in the dark.

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding

Source:

Kosumoth the Hungering in Eye of Azshara (open world).

Notes:

The unsaddled version of this eerie, jellyfish-like mount has actually existed since Cataclysm, and originally served as a mount for Faceless NPCs. Now with a saddle, this is the first time it's been available to players.

To obtain it you first have to unlock the World Quest for Kosumoth the Hungering, which involves clicking orbs at various places around the Broken Isles (killing Kosumoth the Hungering without unlocking the quest will not award you the mount). The procedure is too involved to document here, but you can find all the steps in Warcraft Secrets' guide or the comments section for Kosumoth the Hungering on Wowhead. Check out the WoW Secret Finding Discord for other mysteries they're working on.