"Tripehound" was a nickname given to the Sopwith Triplane, a fighter aircraft from the First World War, used primarily by the British Navy. I've been a big fan of the Red Baron WWI flight sim series practically ever since I possessed the cognitive skills to play video games, and this remarkable machine was the only thing in which I could keep a British pilot alive during the Bloody April period.
I originally picked the name over ten years ago for use on the IRC dalnet servers, and I've continued to use it in numerous web communities namely for its novelty. In retrospect, sometimes I wish I had come up with something more extraordinary, but hey, tripehound wasn't too bad of a choice for an eleven-year-old. Besides, I've always appreciated having a username without a random string of numbers affixed to it.