In the popular 1998 Nintendo Game Boy titles "Pokémon Red" and "Pokémon Blue," the aim was to catch, train and battle various types of Pokémon (or "pocket monsters"), of which there were 150 total. You could evolve them into other stronger Pokémon, sync up your Game Boy with another player's device and swap the creatures.
An article in Expert Gamer magazine issue No. 58 detailed a difficult method for evolving a particular Pokémon into the character Yoshi from the Mario line of games. Two players, one with the "Pokémon Red" version and one with the "Pokémon Blue" version, would both have to beat the game, having attained all 150 Pokémon. The "Blue" player would get the character Dratini from the "Red" player, evolve it into Dragonite and trade it back. Then the "Red" player would go to the basement of the Unknown Dungeon, surf to the location where Mewtwo was found, take a Fire Stone and use it to evolve Dragonite into Yoshi. The article was complete with screen captures showing the transformation, and the new Yoshi Pokémon's designation as number 999.
That Expert Gamer issue was, of course, the April 1999 edition, and the evolution into Yoshi was an April Fools' joke. But the supposed cheat continued to pop up on message boards for years to come.
A similar rumor was spread that you could evolve Lickitung into Luigi by capturing it in a type of Pokéball, holding your Game Boy upside down and giving him a Rare Candy, but this, too, was an April Fools' joke, this time perpetrated by an official Nintendo website, no less.