"Searinox" is a dragon that exists as part of an optional quest in Warcraft III during the second Human campaign mission. Arthas and his band of newly-acquired Riflemen kill him, take his heart, and turn it into a fiery weapon enchantment. I signed up for a Warcraft forum using the name "SearanoX" (spelled differently due to probably a lazy memory and because the large X looked cool to my thirteen-year-old brain).
However, that's not the whole story. I actually hate the name "searanox" (as I capitalise it now). I started using "sear" casually fairly shortly after, and someone on an IRC started using "sea" as a short form, which I have now adopted as my primary handle. The reason I still use searanox is simply name availability - sometimes sea or sear is simply not available, and I can't use special characters like dashes (i.e. sea-) to get around that, so I revert back to the old one, which is almost guaranteed to be available - I don't think anyone else uses the name (I was impersonated once on another forum by someone, but that's it).
I simply haven't come up with another name to use. What I have is familiar and it always works, so even though I'm not the biggest fan of it, since it is unique it tends to be recognised by people where a more generic name would slip through the cracks. I'm not sure how I would create a new handle at this point - it's almost like creating a new online identity (and a simple spelling change or modification doesn't really work with mine, unlike some usernames). My LIVE profile name is RideTheGallows, from an Einherjer song, but that was again due to a lack of availability. I've tried a few Norwegian and Swedish words in the past, like "Krig" or "Rov" but they don't quite work for me.