If there happens to be no lore on it then either I or one of you creative individuals should write what you think happened
I think I will use the Kain and Abel archetype for this Fan fiction

An interesting proposition I assure you. I'm not that great of a scholar in Elder Scroll Lore, Lycanthropy included, so of course I used the wiki. And though there is some information on it, your exact question on how the first were created seems to have no answer. Maybe a worshiper of Hiricine was the first intended victim and it spread from there? It would an interesting tale for sure, the Cain and Abel archetype would also work nicely. I wish you luck!

Maybe a Dr Jekyll and Mr Hyde would work too..

Pssh. Cliche. Could work pretty well actually, though personally I would like the Cain and Able, and it would probably be easier to develop the characters with all that conflict emitting from them. I just think it would be easier to write a Cain and Able type opposed to the Jekyll and Hyde one, though anything is possible I suppose.

There is no lore on how the first Lycanthrope was created. We only know of vampires. So feel free to write about it in any way you see fit.

This belongs in the lore section.

no...its going to be a fan fic trying to put some lore where there isn't......it's not TES official...and is my opinion......the Lore section is for people who want to discuss what already is solid lore created by Bethesda and its implications....Fan fic...are a little lore but don't revolve around it and aren't limited by it

Sorry, I thought that's what you were looking for. I didn't know this was going to be a fan-fiction.