Discover magazine's April 1995 hoax might not have fooled every reader, but it probably gave a lot of them nightmares.
In a brief story, the magazine described the recent discovery of a new life form in Antarctica. The hotheaded naked ice borer was a "hairless, pink, mole-like creature," about six inches in length, with "sharp incisors" and a "bony plate on their forehead." Its unusually high body temperature, radiating from that bony plate, allowed it to quickly tunnel through the ice and sneak up on unsuspecting penguins from below. After a pack of these charming creatures finished off a penguin, "webbed feet, a beak and some feathers" would be all that remained. They may also have been involved in the mysterious disappearance of a 19th century polar explorer, but that was purely speculation.
Despite ample clues to its inauthenticity, the hoax seems to have taken in more than a few newspapers and TV stations.
In a generous 2010 tribute on the National Geographic website, writer Carl Zimmer called it "arguably the finest science-themed April Fool's joke of all time."