There were no Pokemon hacked before released. Technically, Diancie, Hoopa, and Volcanion had to be modified within the game (since there was an open spot in the pokedex for them). The only reason that 719/721 Pokemon were excepted, because their weren't released at that moment. Therefore, it didn't count. Other generations work the same way.
In GEN. 5, people hacked victini before his event came out, therefore they had an early start. Few months later, it was release - since it was already part of Unova's pokedex. Genesect was also distributed at the beginning.
The only Pokemon that was believed to be hacked was "Hoopa Unbound". The prison bottle (found in a pokemart with hoopa), can turn hoopa into his unbound form. But, the only way to get hoopa was via. hacking. Therefore, media has said: "A Legendary Pokemon That Only Hackers Own".
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Hope I helped! :)
