Nightmare Bell

Source
Nightmare Bell has a chance to drop from Fel-Touched Pet Supplies.

Pet Journal
Once a holy artifact used by Draenei priests to calm spirits, it has become animated by anguished souls and drawn to locations of death and corruption.

Trivia

 * This pet is likely a tribute to the Bell of Dethmoora from the removed Warlock questline that rewarded the ability to summon a Dreadsteed.