Until a few years ago, I had a different username for everything. It got really hard to keep track, so eventually I picked one username for all my sites. I realized it was a horrible username later, and since it stinks I won't say what it was. Of course, now I was stuck with it on many websites. To start over, I wrote down all my username ever, and realized they all were too common, and would probably be taken already on alot of sites. So that meant to make a new, unique one that could be slightly edited if taken.
When I wanted to make a pony-related username, I chose my favorite type of pony (pegasus) and just stuck immortal to it. I liked it, so now it's my handle.
I take the mathematical approach to everything, even my handle.