Been assembling a lance of Battletech mechs dubbed "The Four Horsemen", after The Four Horsemen of the Apocalypse. Meanwhile, I needed a name for my e-mail account.
(gears spinning)
And I came up with The Fifth Horseman. As I had to replace spaces with underslashes (_), it changed to the_fifth_horseman. I liked it and so it stuck. Pretty unique too - so far I found only one or two people using that nickname at all.
k:
This also has a good advantage of many derivatives - fifth_horseman, fifthhorseman, tfh, t5h etc, all of which can be used if a short nick is needed,