You cannot tame a jelly fish but you can kill it and use it's wonderful biotoxins as narcotics to tame other huge animals

Their attacks shock you (which makes you unable to move), and they often stick in groups. If you get surrounded by these, you're basically dead. Don't use melee weapons, use a crossbow.

I had one sarco and a tintoberry next thing I know I’m flying on the back of one of these

They drop bio toxins, which can be used for a new tranq dart that's more powerful AND shocks the target.

Early game it’s easy to kite them onshore and harvest the bodies for biotoxin, which makes taming other creatures much easier