What was the planned but unused evolution of Pikachu called?

Gorochu was the planned but unused evolution of Pikachu.

During development of the original Pokémon games, Pikachu was intended to have a further evolution after Raichu. That proposed Pokémon was called Gorochu, but it was removed before release, partly because of cartridge-space limitations and the need to fit the game's data onto the Game Boy cartridge.

The final games therefore ended Pikachu's original evolution line with Raichu. Pichu was added later as a pre-evolution in Pokémon Gold and Silver, creating the three-stage line familiar to modern players: Pichu, Pikachu and Raichu.

Gorochu is not an obtainable Pokémon, an alternate form or a regional variant. It remains a development-era concept, which is why it does not have a National Pokédex number.