If a player trades a Raichu for an Electrode with the Scientist on Cinnibar Island, they will erroneously state that the Raichu evolved when spoken to after the trade. This is due to the text being based off of the Japanese version of Pokemon Blue, where the player trades a Graveler for a Kadabra instead. This mistake was fixed in Pokemon Firered and Leafgreen.
OR does Riachu have a secret evolution that Big Pokemon is keeping secret from us?
Actually, Pikachu was originally intended to have a third evolutionary form called Gorochu (from the Japanese onomatopoeia for thunder,) with fangs, horns, and an overall more intimidating look. Once Nintendo/Game Freak realized how popular Pikachu was becoming as a mascot for the series, they decided to cut Gorochu to keep the Pikachu line cute and approachable.
I knew Big Pokemon was keeping the good stuff from us.