I came up with the name of "Megamariofan" because it describes me well. The only other online account I had at that time was Webkinz, where you have to have a number at the end of your username. I thought it would be the same here and that I needed a number at the end of it. So I chose 65 because the digits are right next to each other on the keyboard and it is easy to remember (being one over 64 and all).
And then I forgot my password (I could of sworn I had written it down too), so I had to get a new username. That was when I was obsessed with M&L: BIS so that's how it became Fawfulfury65. Fawful has fury. :P
It actually has 3 origins: Yamilla Hoshi, like my real name is.
YamiHoshi also derives from "闇" and "星", and I know Garlic, it will be read as "YamiBoshi" if they're placed together.
And we have a website, which adds ".nl" and ".tv" behind it.
Short answer: It's a shortcut from my real name, when I learned some Japanese, I gave it a meaning to it, and we made a site, using the meaning version.
Ironically, my normal username, "SolarBlaze", didn't come from the Mega Man series weapon; it's actually Sonic the Hedgehog-related. Back in 2005, before Sonic Rush was released, I assumed that Blaze the Cat's super form was going to be called "Solar Blaze", but it instead turned out to be "Burning Blaze", but I guess I wasn't too far off. It wasn't until two years later that I began using it as a screename.
Since my screen name isn't my real nickname I will tell about the two.
DyegoHalliwell_IceMario: It's pretty explains it by itself. It's the combination through a '_' of my typical nickname, which is my name with an y intead of i, and the last name of the characters of my favorite series, and my favorite Mario transformation (ignoring the fact I love blue and light blue)
DHM13-97011: My first "coded" name. The letters DHM come from Dyego Halliwell Mebarak, which is the mentioned and the last name of Shakira. 13 it's my age, the '-' just separates it from 97011. 97 are the last two numbers of the year I was born, which is 1997. and 011 comes from 2011 to add an useless actuality touch :P
Not much of a story to my name. "Velli" is short for "Vellisia", the name of a character I came up with for a book of mine that I wrote years ago but still haven't gotten published (see the deviantART link in my sig for pictures of the character). She happens to be a dragoness (the one in my avatar, to be precise) and I happen to identify with her, so I started more or less using her name as my own. Why I tend to pick "Vellidragon" as my name rather than simply Vellisia, I really don't know; I guess I wanted it to be clear that I'm a dragon even in places where I can't use an avatar.
...Well, my Mom and Dad just gave me a name and... boom.
Way back when my username was "Super paper luigi", but someone else decided to use "Super Paper Luigi", and I didn't really want to share a nickname so I changed it to Paper Luigi DS, which changed to Master Crash. Master Crash just sounded cool.