Monsta Mack and Mafia are actually two indy wrestlers that were on a tag team called the hit squad. My friend IRL said he was gonna pick Mafia for his nick on CAG, so I picked Monsta Mack.
My other two nicknames are Jagr (which I usually spell backwards now: Rgaj) and Kefka. Kefka is way too common but I used it for my Ultima Online character. Jagr wasn't too common but I used it for about 6 or 7 years on Kali, so it had to be put to rest. I'm sure most people know where Kefka and Jagr come from. I also go around as David_Blackrose, Blackrose came from my favorite type of rose and David is my real first name so I said what the heck.
My last original nickname was Dwayne_McDohl. McDohl came from the heros name in Suikoden 1 and it really stuck to me, especially since I loved the game greatly (till Suikoden 2 came out, then I went to RPG heaven). Dwayne is actually The Rock's real first name, so I picked it for haha's.
My other nicknames I use to go under as: Sourpuss (...), DarkDave, LightDave, Freeze, Freezer, Ash Ebonysoul, Tenma (from one of Akuma's finishers) and a bunch of other nicknames I can't remember.