It could be hacked but I can't really be certain unless you give me more information such as it's IV's, Shininess, Moves, OT, Original Trainer Name, Where is was Obtained, The Date, Location and Origin Game.
Advice:
It could be just a glitch but if it is hacked don't get rid of it because if it was that much of a problem then it wouldn't of made it through Pokémon Bank.
Possible Solution:
You could try changing Shaymin's Forme from Land Forme to Sky Forme as that should hopefully change it's Ability as well. You can change Shaymin's Forme with the Gracidea Flower.
>In Pokémon X and Y, Gracideas are featured in one of the paintings on display at the Lumiose Museum. A female NPC located at the Snowbelle City Pokémon Center will share one of her Gracideas with the player if she is shown a Shaymin.
