The Swamp Monster was the main antagonist in the book How to Kill a Monster.
The Swamp Monster is a creature that lives in the swamps of Georgia. It is a green-furred creature with the head of an alligator and a gorilla-like body. It had invaded the house of Gretchen and Clark's grandparents causing them to lock it up in a hidden room.
Gretchen and Clark find the Swamp Monster in a hidden room. The Swamp Monster is concentrating on eating pancakes and doesn't notice Gretchen in the room. She turns to leave and he spies her. Dropping the pancakes on the ground, he trounces after her. Gretchen narrowly escapes the locked room, only to encounter Clark in the hall. The Swamp Monster escapes out into the hallway in pursuit of the children. The children run away in fear, dragging their dog Charley with them, who keeps trying to turn around and confront the monster. They make it down to the second floor and lock Charley in a bathroom to keep him safe. The children attempt to get their grandparents' attention, but can't seem to find them in the castle.
The swamp monster proceeds to make it down the floors of the castle as Gretchen and Clark find letters that give them info on how to kill the Swamp Monster. Gretchen has a plan. The stairwell to the third floor is missing some planks as the children run up the stairs and then jump onto the banister, the swamp monster will keep running and fall three stories down to the ground floor of the castle. The swamp monster survived that attempt.
For the second attempt, Gretchen pours drain cleaner, rat poison, ammonia, and turpentine into the pie. As the monster makes its way to the kitchen, Gretchen throws some orange paint and mothballs into the pie for good measure. The two children hide under the kitchen table as the large Swamp Monster makes his way around the kitchen, sniffing at appliances. The Swamp Monster rips the oven door off and throws it across the kitchen. He retrieves the non-poisoned pies from the oven and shoves them whole into his mouth, then turns around and polishes off the third, adulterated pie. The two children huddle in fear as the Swamp Monster stands inches from them, licking the pie plate. Then the Swamp Monster's eyes pop out of its head and it falls to the floor, dead. Gretchen does not want to get too close but it looks like the monster isn't breathing.
The monster later appears alive in the kitchen doorway. Clark thinks it's going to eat him first since he threw the comic book at its head. Gretchen rightly reasons that the monster will likely eat both of them, since they did try to kill it and all. Gretchen, holding the shovel in both hands, attacks the swamp monster in the stomach with the tool. The Swamp Monster picks the shovel from her hands and tosses it behind him back into the kitchen. Gretchen tries to grab the other shovel from the wall of the room, but the swamp monster spots her movement and grabs that shovel as well, breaking it in two with his hands. Gretchen tells her brother to open that second envelope from the fridge. Before Clark can comply, the Swamp Monster grabs Gretchen's arm and pulls her closer to him, her face now in front of the Swamp Monster's rank alligator-like mouth. Charley the dog runs to the rescue, biting into the Swamp Monster's leg. The swamp monster kicks his leg backwards, sending the dog flying across the room back into the kitchen. The swamp monster pulls Gretchen's head into his mouth and extends his tongue, covering her entire face with the Swamp Monster's saliva as he licks her body to prepare for eating her. Before the Swamp Monster can close his jaws around Gretchen's head, he stops, removes her from his mouth and asks in English "You human?" Gretchen confirms she is. The Swamp Monster then bellows "I'm allergic to humans!" The swamp monster dies for real this time.
The second letter that Gretchen reads state that the Swamp Monster had many brothers and sisters and that they come out at night.
- The design of the Swamp Monster is later shared with the Shaggedy from the Goosebumps Most Wanted book titled Here Comes The Shaggedy. The only difference is that the Shaggedy has green scales and brown fur.
- In the Goosebumps Monster Survival Guide, Slappy's Revenge, and the Goosebumps Activity Book, the swamp monster is accidentally referred to as the Mud Monster from You Can't Scare Me!
- The actor who plays the Bog Monster is married to the actress who plays the Snake Lady.
- In the TV adaption of the book, the Swamp Monster (performed by Tim Rykert) had a different appearance. Gretchen figures out that the monster is allergic to humans when Clark accidentally sneezes on the monster (because of allergies).
- The Swamp Monster makes an appearance in the 2015 Goosebumps movie performed by Nate Andrade (who was credited as "Monster #1"). He is one of Slappy's monster and villain henchmen and is referred to as the Bog Monster during the Comic Con appearance. His appearance, however, is different. While in the book he is described as a furry creature with an alligator head and a gorilla body, the movie version looks like a giant creature made of moss.