I named my Venasaur something that I didn't like. Can I re-nickname him? And if you can, where?
(P.S. This is in White)
Yes, unless he was migrated from an earlier generation (III-IV). Just go back to the Name Rater in Castelia City and speak to him and change it.
This is what the lobby looks like. The name Rater is the Old Man at the bottom.
Yes you can in Castelia City, there is an old man who will change the nickname of your Pokemon. He can be found in a building to the left of the entrance to Route 4.