I have a bunch of them all over the internet, which tends to confuse people who are trying to figure out if they've got the same person from one platform to another.
Scarlet Cryptid: (Tapas, Instagram) This one came about because my favorite color is red, and back in high school I had a character named Scarlet. On top of that, I'm one of those people who accidentally sneaks up on everyone, and can disappear into the background in an instant, and I have vague goals of lurking in the woods like a cryptid someday and maybe inspiring my own local legend. It also seemed like an interesting, somewhat approachable 'art name', since my actual given name is so common that trying to find ANY variant of it to use as a username on most platforms is just... not happening.
Toxinfox: (Discord, Tumblr) This one's pretty simple-- I like foxes and I have an armchair fascination for poison. Plus the alliteration of the name is pleasant, and it shortens nicely to 'Tox' or 'Toxin'.
Pointy-Eared-Fiend: (Deviantart) ....Anyone who knows me knows that I have a lot of characters with pointy ears. Plus a lot of other usernames I tried were taken.
KhaosDaughter: (Various chat messengers and forums) I have a superhero character who goes by the alias of Nyx (Which, given her powers, makes zero sense, but I can't change it now...), but if you've ever tried to get any variation of Nyx on any platform, they are allllllll taken. But in Greek myth, Nyx was the daughter of Khaos, so... Khaos Daughter. Plus it just kind of sounds cool, and is virtually never taken.