The "Pikablu" legend is an urban legend revolving around the 1999 role-playing games Pokémon Gold and Silver.
Description
Shortly before the release of Pokémon Gold and Silver, several new Pokémon began to be shown in promotional materials, including a short film that premiered with the release of Pokémon: The First Movie. One of these Pokémon was Marill, a Water-type (later retconned to Water/Fairy-type) Pokémon that slightly resembled series mascot Pikachu. Due to its English name not having been released at the time, fans began to refer to Marill as "Pikablu" due to its coloration. Speculation arose regarding its relation (if any) to Pikachu. Most hypotheses claimed that Pikablu was somehow an alternate evolved form for Pikachu that would be Electric/Water-type. This, of course, was later disproven when the English name and typing were revealed.
Trivia
On the Topps trading card sets, before Marill's official English name was revealed, the description would call the Marill a Pikablu.