I can't imagine that somebody would hack something like this Swellow considering how easy it is to just breed it. That being said, there is no way to be 100% sure if it's not your own Pokémon, but a hack like this would at least be unusual.In fact I was the one who leveled up to level 100. It came to me with high level but since it was back when I just started to play XY I was using it for battling. It's okay to trade it for a regular Heatran then?
EDIT: The item "Destiny Knot" makes it very easy to breed 5IV Pokémon. Just search/google for "Pokémon breeding" + "Destiny Knot" and you should find your answer really fast. I'm to tired to explain in detail and this doesn't belong in here anyway.
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