The planned unused final evolution of Pikachu was called Gorochu.
During development of the first Pokémon games, Pikachu was designed as part of a three-stage evolutionary line. Pikachu could evolve into Raichu, and the developers planned another evolution after Raichu named Gorochu. That species did not appear in the released games.
Gorochu was removed during development because of cartridge-space concerns. The original Game Boy had severe storage limitations, and the team had to cut or simplify content to fit the game. The unused concept is therefore part of Pokémon development history rather than part of the official playable Pokédex in Generation I.
Pikachu’s evolutionary line was later expanded in the opposite direction when Pichu debuted in Pokémon Gold and Silver. This created the familiar Pichu–Pikachu–Raichu line, while Gorochu remained an unreleased design. Fan discussions sometimes confuse Gorochu with unofficial names or rumored Pokémon, but Gorochu is the documented name associated with the cut evolution.