Pikachu's allowed.
Heck, in a couple of continuities, Shane gets a Pikachu who can use ice-type moves (and which he'll usually name "Raichu") and Kim Tyranto (Shane's fictional girlfriend who's a princess of a planet nation called "Draconia") a Pikachu that can use fire-type moves.
The Purple Karjam (one of my characters in the games) has a Zoroark named Loki that was often used to retcon away stuff. (Although nowadays, he doesn't appear often.)