I received a mewtwo today. I have heard before that hacked Pokemon can mess up the game and produce bad eggs. Is this true? Do I have anything to worry about?
If you are afraid it is illegal, check it out on PokeCheck.
If it is legal, you have nothing to worry about! Yay!
If it is illegal, there is a very slim chance of it messing up your game.
Source: Experience with hacked Pokemon and here.