I was trading trough GTS negotiations but when I and another guy tried to trade it just said that one oof the Pokemon could not be traded, why is it so? (I was trading shiny Zekrom for shiny Heatran)
I'm guessing it was because of Shiny Zekrom.
The GTS Negotiations does not allow Event Pokémon or Hacked Pokémon to be traded.
The Shiny Zekrom is most definitely hacked as Zekrom is coded not to be Shiny in the game, so you cannot obtain a Shiny Zekrom legitimately as none have been released by event.
Hope I helped :)
Either one of the Pokemon has a ribbon on it or one of the two Pokemon are hacked. You can't trade those Pokemon.