In short, my username is my IRL name, but it took me a while to actually start using it.
In the ages back when I first began to register accounts on the internet, my typical screename was agentdennis007, from a mission in a Worms game called, "Rescue Agent Dennis."
As I went on, I didn't like how typical the name sounded, so I later changed it to Agent Silverfox. Blue is my favorite color, but Bluefox didn't flow, so I used my second favorite color, silver. I have no idea where the Fox came from.
Onward, I began to think that Agent Silverfox was too long, and I began to recognize how uncommon the name Rex has been throughout my entire life (I've only come into contact with two people with my name), as well as how awesome of a name it is. The "Agent" tag also sounded childish, so I carried on the silver and created Silver Rex.
Eventually, I lost the Silver somewhere, and, simply, Rex is what I now prefer and what I have used as my username everywhere ever since.
Unfortunately, Rex is also taken on many sites, so when I registered for Smogon I had to add underscores. The actual "Rex" on this site has no posts and registered four days before I did. Needless to say, I was pissed off.
It'd be simply Rex if I could, but now its just _Rex_.
/story