It must be a side quest in Celestia then. I'm not sure the specific name of the quest, but it's one of the NPC's in base camp that gives you the quest and reward of the jellyfish. I believe I got it after completing all of the quests in the Floating Land.
It's from a side quest in District of Stars, I don't remember which npc exactly, but I do remember turning the quest to the Archivest when I got the pet. It is not a Floating Land pet. I have not done one quest in Floating Land yet. :P
