Like many people, I used a pet's name. My cat was Mudcat. Without getting into why the cat's name was Mudcat - which is a story in itself - I believe what happened was I tried to sign up at Covers around 2002 and Mudcat was taken, so I used Uncle-Mudcat.
And I used Uncle-Mudcat at a few places for a few years.
Anyway, when they asked me to be a mod when SBR opened in 2005 - that's right, I was an original mod at SBR - I decided to go back to the simpler, more elegant Mudcat. I was like the 5th person to sign up there so no danger of it being taken. Good thing I didn't want the name SBR_John.
I have subsequently used Mudcat at a number of places with varying amounts of activity. It is not unusual to find the name taken though at busy places. I have various plan B handles out in the world.