All you need to do is to do the last two bosses of Heroic or Mythic Plaguefall completely solo. That will cause a Curious Slime Serpent to spawn in the area near the third boss. Petting Curious Slime Serpent grants you the mount.
The first person to discover this was famous soloing Hunter Durendil, which got the mount on accident after soloing Mythic Plaguefall for an upcoming video.
Continue reading »