My username is tied to an NPC from an AD&D campaign dating about 25 years or so ago. Prior to using Miskie, I used another AD&D character name, Chronos - Back when the internet was a few colleges connected together via VAX machines, and messageboards were dialup BBSes local nerds operated from their bedrooms, it made sense.
Once the internet started to become publicly consumed, and browsers went from text based interfaces to the graphical point-and-click devices they are now, the number of people using Chronos as a handle mushroomed.
So I switched- sometime around 1996 or so.
I have also used other handles in the past - to keep my personas separated. Back in the days of USENET, I used to mole and troll liberals under the username NoSoup4U. And back in my Commodore64 days, I managed to circumvent !VMax! disk based copy protection on several 'single load' games (games that never refer back to the disk once they load up) - I wrote my own loader program in 6502 assembly for those games that would display my username as !DaveCopy! as the loader did its thing.