Pokemon developers in a recent interview:

In the game Pikachu only evolves to Raichu, but I hear that at first there was three stages. Why did it end up being only two stages? (Or one evolution)

Nishida: Yes. It was “Pika(chu),” “Rai(chu),” and “Goro(chu).” The Pokémon known as Gorochu bared fangs and even had a pair of horns.

Sugimori: There wasn’t any problem with its appearances, but it was omitted due to matters surrounding game balance.

Read more at http://www.siliconera.com/2018/05/03/pikachu-originally-second-evolution-called-gorochu-large-fangs-two-horns/#LoB2CQczSgDCZZ52.99

Source:

http://www.siliconera.com/2018/05/03/pikachu-originally-second-evolution-called-gorochu-large-fangs-two-horns/