My first space marines were ostensibly Dark Angels (because they were green and I'm not really sure what I was up to). But it wasn't long before I read a bit of fluff in the 2nd ed. Ultramarines book about the Grey Knights, a Cursed founding chapter. Cursed founding sounded pretty awesome (now my marines are Lamenters, I guess some things stick with us).
This was before Grey Knights were well known as the daemon-fighting chamber militant of the Ordo Malleus, so I just thought they were:
and 3) because there was a picture of some ultramarines next to them and one of them had a medusa head on its shoulder pad; had a medusa head as their chapter icon (I was like, 11, give me a break)
Anyways, this led me to repaint my marines grey and black with red bolters and bright green (Scorpion Green (now Moot Green) to be precise) eyes and chapter markings. When I looked through them this morning they were too hideous to photograph, so I'm going to spare you those old models. But to continue my story, one day it became apparent that Grey Knights were already kind of a thing and they totally were not my radical Cursed Founding guys in grey power armour. Thus the Sons of Medusa were born (which I'm sure most of you know turned out to also already be a thing).
Anyways, since they already had skull decals with painted serpentine hair on their shoulderpads, I renamed them the Sons of Medusa and they had a fresh new backstory about how they pursued some Chaos heretics into the Eye of Terror and were forever cursed to bear the blight of the medusa. It was actually pretty cool.
A couple of years later a new Space Marine book came out and I flipped through it and saw this crap (see right). Who are these impostor Sons of Medusa? I felt betrayed. But I was all like "I'm not changing because they're wrong". So mine remained the Sons of Medusa who had nothing to do with those puce green jerks.
So I told you that story so I could tell you this story (one of my friends says this all the time, you can hear him telling stories on Storytime with Blake and Highcove at iPodcast Magic Missile). Many years later, after I'd left the Imperial fold to hang with the Tyranids (and sometimes Space Orks), I came back to my Sons of Medusa with a pack of Naga (from some game or another). I crafted the Snake Marines.
There are only two of them. I guess I'd count them as Space Marine bikers if I had an army to go with them. But the moral of this story is that even though my childhood wacky creativity with total disregard for sense was constantly crushed by real fluff, in the end it resulted in what I think are some pretty cool conversions and maybe even someday the basis for a kinda neat army (not that I'm likely to build it, but you never know).