Pikachu, Raichu, Pikablu
Short Version: With the help of a Water Stone and some mystical chanting, Pikachu or Raichu could evolve into a new water-type Pokémon called Pikablu
Why We Believed It: Because kids in Japan already had him and we saw him in Pokémon: The First Movie
In the days leading up to the release of Pokémon: The First Movie in America, Nintendo was also preparing the launch of the Japanese version of Pokémon Gold and Silver which would be coming out a mere 11 days later. Naturally, press materials, screenshots, and other images of the new Pokémon had started surfacing in various magazines and online, but most American kids had no idea that a sequel to their favorite game was coming and so when they saw this stuff, they immediately assumed they were just unfound creatures within the games they already had.
Pikablu (real name: Marill) was the most popular of these new Pokémon thanks to its similarities to Pikachu.Both being mouse-like in appearance, people assumed they were related in some way and so rumors of a hidden evolutionary form, Pikablu, surfaced. Other Gold/Silver exclusives got their own rumors too, especially ones with origins in Red/Blue like Slowking and Steelix.