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. 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.