Pikablu
YES!! Everyone has been asking me about this one. Pikablu is one of the most interesting cheats I have ever seen. And it goes way deeper than I thought it did.
To get it out of the way, yes, I know Pikablu is called Marill now. But, SHE IS IN THE ORIGINAL GAMES, and she is named Pikablu in those. That's what we'll be talking about here. Deal.
Anyway, none of that nonsense about Tauros. You have to catch all 150 Pokemon and get the diploma from Celadon.
And now here's the scary part... you have to start a new game. D: Yeah, I know. I didn't wanna do it either. But my friend told me this is the only way. Transfer them to Pokemon Stadium if you must, I kept my beloved Charry. Once you have the new file, play like normal. When Prof Oak offers you the starters, look at each one individually and say no to all of them. Now he will show you his secret collection!
Before you ask, they are all level 6 because I level up while feighting Gary.
I think maybe they can evolve, but I haven't tried yet. I would guess they evolve with the Water, Leaf and Fire Stones?