asked Blubby "Where did the fear of jellyfish come from for such a mighty dragon?"
"Well, I appreciate being called mighty, even with such a harmless fear... Anyway, I think it comes from... This one time, when I was pretty young, apparently I got myself into a swarm of jellyfish for whatever reason. It was so long ago I don't even remember it happening, my parents told me about it. It somehow stuck with me even after I forgot about it."
(Meta-wise, his fear of jellyfish is actually based on my own. I've got Finding Nemo to blame for that.)