I've been using "Shadsie" as my online handle for years. Here's how it evolved:
"Evolved" being the operative word because I chose my first online handle back when I decided I wanted to write Pokemon fanfiction. This was like... 15 years ago. I needed a username to join a fanfiction site (that's now dead) and I didn't want to choose a fandom-specific one, like "Mew2," which was an author I read. I looked at my little gray cat named Shadow and typed in "Shadowcat." At the time, I had *no idea* that the name was shared by a character in the X-Men comics.
When I found that out in an awkward way (someone saying "Oh, you must be a Kitty Pryde fan!" and wondering of "Kitty Pryde" was a brand of cat litter), I changed my name to "Lady Shadowcat" for a while and kept that for a good long time, until I found other "Lady Shadowcats" on The Fanfiction Network and around the Internet in general. My online friends on the Pokemon / Trigun / General Anime and etc. forums and AIM buddies liked to call me "Shads" and "Shadsie" so I shortened my general username to "Shadsie."
THEN, I play Twilight Princess and find out there's a character named Shad in it. And people have connected the name to my username, assuming that I named myself after him when my username predates his existance in fiction by a longshot. I JUST CAN'T WIN!
Fictional characters need to stop stealing my username! And the overly generic name of my cat, for that matter!
(My cat, Shadow, is actually still alive... old thing. I couldn't take her with me when I moved across the country so she's been living with my parents for the last seven years. I get updates on her life. I miss her).