The Screaming Lobster is an insane screaming lobster in LBP2 that turns other sacks insane, or a meme created by Rathymos_X. This creature appears in Rathymos' Uchaenlova and Filler levels, which don't have any descriptions (apart from Filler's "Why"). In Filler, Rathymos is seen putting a baby in an oven, and out of nowhere, Screaming Lobster appears and turns him into one of his kind, as well as the baby that was going to be burned in the oven.
It is insane and is always screaming and laughing manically, except for when it's not screaming and laughing manically. You can turn into a screaming lobster by being approached by it when it's screaming or laughing manically.
Screaming Lobster one day turned into a Godzilla-sized kaiju without explanation, and it wen off to fight the King of the Monsters in an epic battle. The outcome of the battle is unknown, and it may have something to do with the very small Screaming Lobster running towards the Screaming Lobster Emperor in the picture right above this text.