Lenny is a Mutant type jellymon that has no evolutions. However, Lenny is one of the strongest jellymons out there, and it is rare to spawn.
Here are some of his moves (thanks hash and axo for the moves aswell.) 30: Mega Drain, Nightmare 30: Poison Sting, Shocking Punch 40: Recover 50: Blizzard, Hydro Pump 60: Ye' Ol' Brzap, Brzap
I'd recommend you to buy some JM12 (reinforced jars) for a better efficiency of catching it. Lenny/Big Lenny is a jellymon taken from the jellydex for jellyfishing. In jellyfishing, he can zap you immobilizing you to jellyfish for 30 minutes. The chances of catching it is extremely low, near impossible.
it was first appeared on the episode "I'm your biggest fanatic". In that episode, 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.
In the episode "Jolly Lodgers", A picture of him appears amongst the merchandise at Dr. Manowar's stall. Operation Krabby Patty: He also appears in the "Hide n' Go Jellyfish" mini-game, where he floats around Jellyfish Fields and sends electrical orbs at SpongeBob or Sandy. If they are hit by the orbs, they will get a time penalty.