Impossible by normal means.
Look at Mewtwo's move listing.
Normally, the earliest you can catch Mewtwo is 70. (Might be earlier on some carts)
Still, Mewtwo learns moves really early.
Lots of other Pokemon have this problem, specifically evolutions.
Say a Pokemon X evolves from Y at Level 30.
Lets say you cannot catch X in the wild.
But X learns Tackle at 25.
There's lots of that kinda stuff going on.
A viscous, Psychic Pokemon that was created by a scientist after years of horrific gene splicing and DNA engineering experiments. It's DNA is almost the same as MEW's. However, it's size and disposition are vastly different. It's cold, glowing eyes strike fear into it's enemies. It usually remains motionless to conserve energy, so that it may unleash it's full power in battle.