The Tongue Monster is a monsterfied version of an unknown male. It is tall, with an especially large forehead. When hunting its lower jaw, throat, and upper chest open up vertically to reveal a long tongue. The tongue acts like a proboscis similar to a mosquito's. It stabs the victim and sucks up its inside, leaving the victim's skin as a shell of the person it killed. The force of its tongue is fairly powerful and can break through walls.
The Tongue Monster was the first monster many of the survivors encountered when they investigated why the lobby is locked up. Jeon Song, who was in the security office when the monster revealed itself, opens the front shutter and unknowingly let it inside. The monster shambles inside and immediately feeds on the first human it encounters as the bystanders look in horror. In the ensuing panic, Wook Pyeon manages to tackle the monster back outside the apartment, and Jeon closes the shutter with Wook barely able to make it inside in time. It continues to lurk outside the apartment building.
The monster, or rather its tongue, is later seen when fighting a monsterfied Mr. Kim, the former security guard. The monster heard the commotion inside and was able to use its tongue to break through the building's wall. Hyun Cha uses this to his advantage by attaching it to Mr. Kim. The monster begins sucking Mr. Kim's insides, and lets go after it is finished.
The Tongue Monster is later cannibalized by the Steroid Monster.