Mewthree
From Bulbapedia, the community-driven Pokémon encyclopedia.
mewthree is one of the most famus fake pokémon. it was suposed that when you obtained all the 150 pokemons in th red and blue vertions,you had to go to the place you obtained mewtwo and use rock smash in a rock. then, mewthree Lv 99 will appear. (y dont't make responsable if you want to try this FAKE CHEAT). suppostly, the mewthree's dex information was: " this pokémon was created with both mew and mewtwo's DNA".
