I've once written one fan fiction about the roguelike game "Dungeon Crawl Stone Soup" when I was already over 20. I don't remember other fan fictions by me, so probably it was the first one? 
In this game, there is race "Felid" which is intelligent cat, who can use different sort of magic efficiently. It is nevertheless hard to play, cause cat can't use items, which require having fingers, and can't wear armor, because all armor in this game is for humanoids. But I've liked to play this race anyway cause it is cat. 
So I've written the small fanfiction about such a Felid. He was a cat of a wisard, and one day his wisard have died. And Felid have come into dungeons to find the Orb, because the Orb have immense power, and cat hoped that it will let him to ressurect his owner. But during his trip the cat have changed too much, so...
Well, I maybe should translate it to english some day. 