I have Pokemon White 2 and I wanted to rename my Zoroark. Is it because It was originally N's?
Yeah, N's Pokemon act like traded Pokemon so unfortunately, you will not be able to name it. However, if you want a nicknamed Zoroark you can just breed for a Zorua, nickname it and evolve it.
If It says "N" in the Trainer Name section, then Zoroark is treated like a foreign Poemon and cannot be renamed.
Source: I have tried.
Yes, that's the reason, you can't rename other people's Pokemon :3