Big Lenny is a large and dangerous jellyfish, and the only known jellyfish to kill its victims with a sting.
As SpongeBob reveals 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 escape unharmed, as he has a sore on the side of his face from the sting, which causes pain if it is touched.
Not much is known about Big Lenny since he is only shown as a picture. However, it is shown that he has an evil, humanoid face, and ten tentacles, at least according to Dr. Manowar's drawing. He is also colored pinkish-red.
Big Lenny is obviously a very dangerous creature, because the lump on Dr. Manowar's face is very large and still hurts, despite the passage of time.
Role in series
"I'm Your Biggest Fanatic"
A picture of him appears during Dr. Manowar's talk, and SpongeBob reveals that Dr. Manowar is the only person to ever survive Big Lenny's sting.