first username anywhere was based off part of my first name and part of my last name, so i'm not gonna share that one. made it when i was like 8 because i had no idea what else to use.
in 2005-2006 i used my first real username *invincible* on a site called Bloonsworld, the only site i've seen to this day that allowed * in username field. so i couldn't use that name anywhere else. when they got a new site for their upgraded game, i changed my username to The Real Invincible. i was like 11, don't judge me lol
since like 2008 i've used 2 main usernames (and variations). one is my last name + 2 numbers (i have a pretty uncommon last name so it works) that i use for most personal things where i would use my real name. the other is Decon082 which had its first use either on my first runescape account or on my youtube, i can't remember (both were in 2008). Decon is still what i go by on most everywhere else.
for my twitch i had to use DeconStream because stupid reasons, and i've shortened it to Decon cause numbers aren't really that cool if you can take them out and still have the username available.
i changed my username on osu! to Catgirl in december and haven't looked back~
other usernames i've had on other places that i can remember (these are mostly alts/name changes):
hayai_usagi SpeedyRabbit (follow me on hitbox lel xDxDxD)
Sneasel Chespin Angel of Darkness Angel with a Shotgun (wow i was obsessed with nightcore from these 2 lol)
sadjokjkosadjkosdsdajsdok (best username, yes it actually meant something)
and an assortment of other memes / inside jokes
Ah shiieet. The first time I started making online accounts was when I was 8 and the first online account I made was for Xbox Online, playing Halo. My first username was SiriusBlue because I've always had a fondness for stars, especially blue stars.
I've used a lot of usernames in the past and most of them come from snacks I loved when I was a kid, mostly because they are the first things that come to mind when I make a username, here are some:
I had all these random names from various forums and games which I wasn't really active.. Right now I pretty much only use twistedfantasyy or Twisted Fantasy as a homage to Kanye West's 2010 album, My Beautiful Dark Twisted Fantasy.
I am one of those people who never anywhere change nickname. with 2007 year never at any site and in any game not changed my nickname. thus for 9 years, I not found anywhere in the world, a person with the same nick.