In my case, my usual pseudonym was already taken when I stumbled upon the Monastery, so I settled on my favorite fallback.
Quite a while back, in response to people making typos in my name on an online chatroom, I made the unfortunate claim that I would answer to pretty much anything. Several people immediately decided, due to my penchant for being rather blunt in my interactions, to call me the "rat bastard." The name stuck, though one person for reasons known only to herself, kept trying to change my name to "paco."
I decided to pseudo-latinize it to make it slightly less offensive, and the rest is history. When I'm feeling truly full of myself, I append "rex" to indiciate I am truly the Lord of the Illegitimate Rodents. ;)
-rattus illegitimus rex