Slime Serpent



The Slime Serpent is a mount that resembles a stitched-together serpent of N'Zoth.

Source
This mount is obtained by petting the Curious Slime Serpent located in the slime by the Broken Tower in Plaguefall. The Curious Slime Serpent is spawned by completing the dungeon on Heroic or Mythic difficulty while solo.

Note that the first two bosses can be skipped, but Domina Venomblade must be killed to open the door to Margrave Stradama. After defeating Stradama, use the teleporter to return to the surface to find the serpent. There may only ever be one player inside the dungeon at any one time, or else the serpent will not spawn. However, it is possible for one player to kill Domina and clear to Stradama, then leave the dungeon, and then a second player can zone in (they must be in the same party to share the instance ID). As long as the second player is capable of soloing Stradama, they will be able to loot the mount afterwards.

Mount Journal
''This mysterious creature lurks beneath the slime pools of Plaguefall. No one knows where it came from.''