Fake or Manipulated
The description states
"A fateful encounter seems to have led it across space and time to reach the Kalos region from the Unova region".
Now, there was a Shiny Shaymin Event starting from the 19th of December 2011 to 5th January 2011. This is the Shaymin :
The only matching correct detail is the OT ''PC'' and travelling across from Unova.
As Blobyolo mentioned in the commens above, the Pokémon could've been genned carefully, thus avoiding the 'Hack' Censor. He also mentioned the Shininess and the Event you receive was not a Shiny Shaymin
You said that it was traded and if it was Wondertrade or GTS it literally has FAKE plastered onto it . Event Pokémon cannot be trade on GTS or WT.
Also, during the time of BW ,before WiFi shut down, there were very simple genning programs which you used on your phone. You just build the Pokémon like you would on Showdown ! and it would just arrive at your doorstep via GTS
All other Shaymin Events