I'm not really into that specific genre of fanfic, but you could try searching on popular fanfic websites like Archive of Our Own. They usually have a wide range of FNAF fanfics, and you might be able to find some that match your criteria by using their search filters.
I'm not aware of any specific well - known authors for that particular type of FNAF fanfic. The FNAF fanfic community is large and diverse, but this is a rather niche sub - genre. You could try looking at the top - rated FNAF fanfics in general and see if any of them are in this category and who the authors are.
One interesting concept could be the unique character dynamics. In FNAF futa fan fiction, new relationships between the animatronic characters can be explored. For example, how they interact in a different gender - swapped or modified context. It might also involve re - imagining the horror elements with a new twist based on these new character types.
First, get to know the FNAF universe really well. Know the characters, the locations, and the basic horror elements. Then, think about how you want to introduce the futa aspect. It could be a gradual change in the story or something that's there from the start. Just start with a simple idea, like a day in the life of a futa animatronic at Freddy Fazbear's.
Definitely not a true story. FNAF is a work of fiction that takes place in a fictional world with its own set of rules and scenarios. It's all about creating a thrilling and immersive gaming and storytelling experience, not reflecting reality.