Because he's mentioned in The Silmarillion, as is Radagast the brown it's just Jackson's amped up these characters to fill out the story a little more for it to make a decent trillogy. I personally think Smaug is the antagonist still and this white orc is just like the Orc generals in LOTR.
He's not mentioned in The Silmarillion actually. He's mentioned in the Appendix of LotR when the history of the Dwarves is being discussed and is supposed to have been killed 100 years before the events of The Hobbit.
Winner of the "Halloween" Lit Contest My lit bits Last update January 6, 2014