The Jellyfish is a relatively unique enemy that can be found early in the game.
Once a jellyfish finds Albert West, it will follow him slowly. Unlike every other enemy in the Mars colony, jellyfish are capable of opening doors, although only doors that must be interacted with and are not locked. If it gets into melee distance, it will attempt to stab him with a tentacle or two which will drastically drop West's stamina, slow him down for a second, and heavily damage his suit.
The jellyfish's only attack takes about a second for it to perform after getting into range, so even melee can easily dispose of them. They have the same health as normal mutants, so a very well-placed (and lucky) shotgun blast can instantly kill it on any difficulty. Unless there are nearby doors that you don't want opened, jellyfish should be among your lowest priorities. They are slow enough that, even with other mutants around, they probably won't catch up to West.
What is more threatening than their combat capabilities is their actual existence. Like most enemies, jellyfish are alerted by the sounds that West makes, including unsilenced gunshots and running footsteps. This means that, while you are fighting one enemy, a jellyfish may be alerted and open a nearby door, potentially bringing more enemies into the fight. Once local infection passes 60%, rooms can be packed with enemies (most of which are likely aware of West's presence as well), so a jellyfish giving them all access to West can be an enormous use of resources at best and instantly fatal at worst.