Big Lenny is a large and dangerous jellyfish, and one of the only known jellyfish to actually kill its victims. As SpongeBob revealed in the episode I'm Your Biggest Fanatic, Dr. Manowar is the only fish who has survived an attack from Big Lenny. However, he did not esacpe unharmed, as he has a sore on the side of his face from the sting, which hurts very much if it is touched.
Not much is known about Big Lenny since he is only shown in the episode I'm Your Biggest Fanatic. However, we do know he has an evil, humanoid face and ten tentacles, at least according to Dr. Manowar's drawing. Big Lenny is obviously a very dangerous creature, because the lump on Dr. Manowar's face is very large. Dr. Manowar also screams quite loud when Patrick touches it, which implies that it can still be painful if touched.
Big Lenny has never made a true appearance in the series.
Dr. Manowar's name is a reference to a Portuguese Man O War, a deadly jellyfish.
Unlike other jellyfish stings, people can die if stung by Big Lenny, most likely because of the size of his stingers.
He has never been seen in places like Jellyfish Fields, so it probably lives in a prison, a safekeeping area, or in a jellyfish field too dangerous for normal jellyfish.