In Hero Looper the Jellyfish is a floating creature made entirely of water. Although it does not appear harmful the Jellyfish are a dangerous enemy which once summoned will perform deadly slashing attacks on your character. You should only fight Jellyfish once in order to obtain the Encyclopedia and then never attempt to fight it again.
You will not be able to spawn a Jellyfish in Loop Hero, the only way this creature can appear in the game is if it is spawned by Sirens after Chapter 3. A Siren is able to summon a Jellyfish once every 3 attacks with her summoning ability, and they act like her minions on the battlefield. A Siren's control over Jellyfish is similar to that of the Necromancer's ability to command Skeletons. A Siren spawns once every Loop which consists of a Shipwreck tile.
Although the Jellyfish is only water it still possess a potent sting, as they are minions their defenses are weak and once killed it will drop Shapeless Mass as a resource you can use. The Jellyfish has liquid traits, these include Ambush, Friendly Fire, and Legionnaire.
Check out our How to Get the Broken Geography Achievement in Hero Looper which is unlocked when you place 10 cards you did not have in your deck.