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
)