I remember alot of the RBY rumours. I spent ages scouring Google looking for them all when I was writing a fic based on the Pokegods idea.
There was a Mist Stone that could be found in the Unknown Dungeon which evolved most pokemon.
A Volcano Stone, Lightning Stone, Raindbow Stone, Ice Stone, Ground Stone and a Wind Stone.
Eevee had more evolutions. Volceon, Lunareon, Tropeon, Breezeon, and more.
Charizard became Charcolt
Blastoise became Hydrotort
Venusaur became Sapusaur
Pikachu became Pikablu, Pikflare, Pikabud or Electrachu
Gengar became Spooky
Snorlax became Momo
BUtterfree becane Locustod
Beedrill became Beepix
Nidoking became Nidogod
Nidoqueen became Nidogodess
Mewtwo became Mewthree or MetalMewtwo
Mew became HydroMew or MetalMew
There were loads more I just can't remember them all.
If you took a Dragonite to the exact spot you captured Mewtwo and used a Fire Stone on it it would turn into Yoshi.
I also remember rumours about Luigi being a pokemon.