The most useful monster.

How are jellyfish related to poison, or am I missing something?

Apparently, yes... Because when a jellyfish attacks, it deals damage not only with its sting but also with paralyzing poison, right?

Not poison, but a petrifying gaze, but the area of effect is roughly the same.

Yes, it's a paralyzing poison/gas that doesn't affect undead, mechanical creatures, and elementals. That's how it's described in the game...