Cerulean Cave Mewtwo .

The final boss monster in the game, hiding in an "unknown dungeon", with it's lore being foreshadowed throughout the game.
In an environment where Psychic types are arguably overpowered, with no Dark and Steel -types to resist it, it's only weakness, is a 11 year old kid with infinite Master Balls.
 

Species   Mewtwo   Nickname   (default, in )   OT   (recipient)   TID   (recipient)   Distribution   Cerulean Cave   Location   the Kanto region   Dates   (Various)   PID   Chance of Shiny   Games   VC RBYG

Lv. 70

Nature   Random   Ability   Unnerve (HA)   Item   None   Bundled Item   None

Moves   (ITALIC means Relearnable)   Swift   Psychic   Barrier   Recover

Format Ver.1.0.5-3, Post Updated Date:20170912_1414

Details above depicts one caught by a player, not fully representative of this file.
If transferred from VC GS, it'll instead state the Johto Region.