An ugly or fat girl.
A person who has sex with a fat and/or ugly girl is called a Land Monster Slayer.
If you know anyone who has sex with fat or ugly girls, it would be correct to begin referring to them as a Land Monster Slayer, because they, in effect, defeated the Land Monster at her horrible game.
Guy 1: "Those 2 chicks over are total land monsters."
Guy 2: "Definitely, man. I haven't gotten any in a while. Want to go slay the land monsters with me tonight?"
A person who engages in sexual activity with an unattractive female on a regular basis. A land monster is an unattractive female that resembles a prehistoric beast that somehow managed to make it on land and find a way to a party.
Frat Guy 1: "Dude, Max hooked up with another fat girl last night. He does that everytime he gets drunk!"
Frat Guy 2: "Good for him. He's definitely a Land Monster Slayer."