TMR said: Mewtwo shouldn't be there. it may be legendary by game standards, but it isn't in the true sense that there were never any legends about it
Mewtwo could be considered a modern-day legend; a legend in the making, if you will. Rockets probably heard whispers of a powerful Pokémon created in their labs and how it escaped. People will believe, rumors will get out of hand, sightings (true or fake) will be reported, theories of what it's done and what it's doing now will be discussed...