It was a skit originated by Chevy Chase circa 1976 on Saturday Night Live.
The land shark was an intelligent shark that walked on land and ate housewives in their houses, by disguising itself.
Here is an example as it was written on a website which includes a picture of the land shark skit:
http://www.snn-rdr.ca/snn/old/nov98/nov98/snl.html
Someone knocks at a woman's door.
Lady: "Who is it?"
Land Shark: "Plumber."
Lady: "I didn't hire a plumber. Who is it?"
Land Shark: "Flowers."
Lady: "What...for who?"
Landshark: "Plumber."
Lady: "You're that crazy shark, aren't you?"
Landshark: "No, maam. I am just a dolphin. Will you please let me in?"
Lady: "A dolphin. Well, O.K."
She opens the door and the land shark swallows her.
My favorite land shark skit is as follows"
There is a knock at the door.
Lady: "Who is it?"
Land Shark: "Candygram."
Lady: "No, it's not. You're that land shark."
Land Shark: "No, I am just a struggling college student-trying to earn some money."
The lady feels guilty: "Well...O.K." and she opens the door.
The land shark immediately swallows her.