I started as Ghost51.  Couldn't come up with a good unique name so I went with "Ghost" because I thought I'd attack people so fast that they would think a ghost killed them, and "51" because I was a brainwashed teen who had been told the P-51 was the "best" fighter of WW2.
Stuck with Ghost for a few tours, added squad number to it, etc.
Banshee7 came about when I had become considered one of the most hated squeakers in the game.  Got caught in the wrong crowd and stuff.  "Banshee" came from three things--1.)  I used to ride a friends Yamaha Banshee all the time.  2.)  The P-51B named "Beantown Banshee"  3.)  It sounded cool.  The "7" came along because of the overused 'lucky #7"