Mine is actually pretty odd since I was (and still am tbh) a weird kid
I used to watch PeanutButterGamer on his second channel where he played Minecraft Hardcore. His in-game name was PBG and I wanted my name to be an acronym since I liked how PBG sounded so I tried to come up with something on my own.
I decided to base it around Koopa since I liked turtles and green was my favourite colour. I didn't want to use "gamer" as a part of my name since even at 12 I thought that would be kinda cringe, so I came up with Guy instead. Lastly was finding a third word to make the full 3-letter acronym and I decided on TheKoopaGuy.
I came up with TheKoopaGuy or TKG at 12 and I haven't changed it since. My plan was actually to go by my real name in the MK community (which is Lucas, if you didn't know), but I remembered that I had registered to mkboards as TheKoopaGuy so I started going by Koopa.