Howler Leech

The Howler Leeches are fearsome, extraterrestrial organisms featured in the Konspiracy Komiks book The Attack of the Howler Leeches (guaranteed to be 37% true!). According to the comic book, the Howler Leeches are coming to attack Earth. The invasion consists of three stages:
 * 1) Somehow the Leeches manipulate Earth's climate to create a new Ice Age;
 * 2) They arrive on the planet in their atomic, blood-propelled U.F.O.s;
 * 3) The Leeches devour all creatures on the world and slowly digest them for 35 years.

When Daggett reads the comic book, he gets terrified, and a snowstorm convinces him that the Leeches are arriving. When his brother Norbert disappears, Daggett believes that Norbert has been eaten by the Leeches; that he (Daggett) is the last Beaver on Earth; and that the Konspiracy Kevin doll is his only friend now. Soon the Leeches appear and Daggett is scared stiff, but it is revealed that the "Leeches" are actually just Norbert and some friends disguised to play a prank on him. Daggett is happy to know that there are no Leeches after all.

However, the 37% truth guarantee is revealed to be real at the end of the episode, when a pair of real Howler Leeches appears, take a frozen dog as a souvenir, and return to their U.F.O.