The monster that Johnny hides away by painting his victim's blood on the wall of his house. At one point in the series, Johnny is unable to paint the wall because of an accidentally self-inflicted gunshot wound. This, and his approaching death, is what causes the monster to break free. This is why one of the doughboy's wants Nny to kill himself, their "master" behind the wall needed Johnny dead in order to liberate it, while the other doughboy wants to keep their master imprisoned so they can be free. The monster breaks free and it goes on a rampage through Johnny’s house, killing many of the victims imprisoned there. Presumably it is the animated cesspool of human negative emotional residue the devil spoke of in his monologue. Fans have named this creature "Moose" because of a caption in volume 5 that reads "Fun Fact: It's not a moose." Also, in a strip in volume 2, Johnny is wearing a shirt that reads: "777 # of the Moose." Johnny lives in house #777. Krik, one of Nny's numerous captives, exclaims that he thought the noises in the wall was a "moose".
The Wall Monster could also be seen as a nod towards famous horror writer, H. P. Lovecraft, due to the fact that the wall monster has no definite shape and seems to be mostly defined by tentacles (similar to some inter-dimensional monsters in Lovecraft stories). Krik also mentions that the noises preceding the monster's appearance make him think there were "rats in the walls", which is the name of a famous short story by H. P. Lovecraft.
The first reference to the wall was in the second "scene" in the first volume, where a surveyor comes to Johnny's house asking questions, one being about a girl found drained of her blood behind a mall, then implies that the blood was consumed by a vampire, to which Johnny replies "I never drank her blood!! You see!! It changes color when it dries!" (pointing to the wall) "It NEVER stays!! I have to keep the wall wet!!" just before killing him.
Invader Zim makes a subtle reference to the Wall Monster in the episode "Room With a Moose," in which Zim sentences his classmates to suffer in a room which is inhabited by an actual moose.