During junior high (what is now called middle school, I don't know why unless "junior" was deemed demeaning by the PTB), a friend and I wrote and illustrated a long tale based on Disney's "Swamp Fox" series. It went on for 2 large-sized loose-leaf binders, and we managed to keep the project going even when she moved away. We'd swap who had custody of the notebooks, and exchange loose pages to be added. In retrospect, we were trying various ways of achieving what Word Track Changes and cut-n-paste do now. Ah, the endless inventiveness of the young!
During those years, "Wagon Train" was extremely popular with my friends. It had so many elements that just begged to be put into our own stories: horses (first and foremost), a handsome and roguish scout, a fatherly wagon master, and lots of characters you'll never see again. You got to go to new places in every episode, but you always came back "home." It was so easy to say, "If I were on the wagon train, this is what I'd do..." I suspect that "Wagon Train" had the same allure that "Star Trek" did, in terms of furnishing such wonderful material for beginning story-tellers. And it had horses... or did I say that already?
I think my writing was a little schizophrenic in those years, or rather, multiple-personality-like. With my friends, I'd write proto-fanfic based on series we all watched, so we knew all the characters, likely scenarios, kinds of Trouble our Mary-Sue characters could get into, and all the usual villains. That gave us a foundation for our semi-collaborative writing. At home, however, I wrote stories that were more like the ones I read instead of watched on tv/film.
In the end, it was these stories, the solo ones that sprang from the images inside my head instead of the ones created by others, that gave be a springboard for my original work.