Remember back in the Pokémon Red/Green/Blue Versions where you were given the option to trade your Raichu for an Electrode? No? Well, that's probably because you were smart enough to recognize a bad trade. Either way, if you go along with the trade, the NPC will tell you that your Raichu evolved.
No, that wasn't a typo. Pikachu was originally supposed to have a second evolution called Gorochu. Unfortunately, it was scrapped due to "balancing issues." I hope this Pokémon is added back into the games one day...
