A plumpkin is a type of animal which is rarely found on Earth. The requirements for a creature to qualify as a plumpkin are that it must be a carnivore, and its mouth must at least appear to make up at least 40% of its entire body. Plumpkins are also often round, and usually are willing to try and eat just about anything. This makes for a creature which is essentially just a head. It is also not uncommon for plumpkins to have small, mostly useless arms.
Most plumpkins are found in fiction, but there are some real-life examples, such as the deep sea anglerfish.
Ted: "It was crazy! It was as big as my TV, and it had this huge mouth! It bit my neighbor's head off!"
Bill: "That's horrible! I didn't know we had plumpkins around here."
Example in Film: The Martians in "Invaders From Mars" (1986)
Example in Game: The Cacodemons in "Doom" (1993)