I originally made mine for an RPG character: Theymos Amastica. It was created without much thought, being only a minor adjustment of the "Thamior" and "Amastacia" names given as elven name examples in the D&D 3.5 Player's Handbook. I began using it for all of my online game accounts, as well, and then I used it even for non-game accounts so I wouldn't have to remember multiple usernames. (At this point most of my online accounts were game-related.)

I sometimes wish that I had chosen something better and more meaningful, though it is nicely unique: almost all Google results for "theymos" are related to me. It would be very difficult to change at this point, after using it for 5+ years.