My name came from a netgame:
I was playing a match against 3 people. One was "Flame" (_the_hedgehog), another was "Shadow" (of MKraces fame), and the final was originally something else, but the name of him I remember was "Xeroshin". I was "Flameshadow", a red Shadow (how original), and noticed that there was a Flame, a Shadow, and a Flameshadow. Xeroshin then piped in ("And one Xeroshin."). I responded that he didn't matter. He might have said he did, I forgot, but I think he didn't say that. I then changed my name to Flameshadowxeroshin ("Now you do."). The alternate "FSX" form was created when I discovered that registering on 64digits as Flameshadowxeroshin would cut the name down to Flameshadowxero. I think I created FSX then, and first used it there or here. The name has stuck, and I still like it, unlike Segmint.