Wowwiki is a special case wiki that moved from it's own hosting to Wikia, they kept the domain name, and got some other stuff that doesn't typically happen on other wikis on Wikia.
You can register a domain name and have it point to wikia.com if you'd like, but remember if you give out that domain you'll have to keep paying for it otherwise people might stop being able to find your wiki.
If your wiki is big enough and there is overwhelming wiki community support for having a custom domain name, Wikia might allow you to have one and they will pay for and maintain it. WoWWiki is not the only wiki that Wikia does this for, I think. Lyricwiki.com works for lyrics.wikia.com as well. Of course they are HUGE wiki and they may have come from a similar situation as WoWWiki (aka were large and hosted elsewhere before moving to Wikia).
As Randomtime said, you can register a domain that points to your *.wikia.com address, but you probably want to coordinate it with Wikia.