Jellyfish are aquatic and transparent beasts that acts as a decorative NPC, they have a luminescent cap and potentially-poisonous tentacles. Most of them are located in deep sea caverns such as Dive Bar and Heyman's Hideaway.
The jellyfish had been released in the Legion expansion pack as mounts and as mobs, and as companion pets in Battle for Azeroth. They are smaller and friendlier than helsquid.