It depends on the Pokémon in question. If it's a good non-legit then nothing will happen but if it's a bad non-legit then it could become a Bad Egg which can ruin your game.
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legit means not hacked.
I've tried to do what it says on u tubebut it didn't workModified by Wyra: There is no legit way to steal a rental Pokemon from the Battle Tent -- "legit" including glitches.
While there is no legit way to combine Pokemon there are various cheats and hacks which allow you to do so.
At this time, there is no legit way to capture him.
Use a legit game and not a flashcart or emulator.