One of the best back stories in FNAF is about the Purple Guy. He was a serial killer who haunted the Freddy Fazbear's Pizza places. He caused the deaths of many children which led to the haunted nature of the animatronics. The animatronics, initially meant for family fun, became vessels for the vengeful spirits of the murdered kids.
The appearance of the animatronics in unexpected places is really scary. For example, when they suddenly show up in the dark corners of the pizzeria in the stories.
The animatronics themselves can be considered main characters in a way. Freddy Fazbear, Bonnie the Bunny, Chica the Chicken and Foxy the Pirate Fox play significant roles. They are the sources of horror, with their own distinct personalities (although they are haunted and menacing) in the stories. For example, Freddy is often depicted as the leader among them, and his appearance is always a sign of great danger.
Some stories might focus on the mystery behind the creation of these animatronics. There could be dark secrets about the people who made them, and how they ended up being haunted. It's all about the horror and mystery that surrounds the FNAF world.
The mystery surrounding the backstories of the animatronics is quite scary. You don't know what really happened to them before they became these haunted - like beings. It makes you wonder what kind of dark secrets are hidden within their mechanical frames. And as you read through the stories, the sense of dread builds up as you learn more about their possible motives for scaring the characters in the stories.
I'm not sure specifically which 'fnaf pregnant fanfiction' stories are extremely popular. However, in general, the more unique and well - written ones that manage to blend the pregnancy concept with the existing FNAF horror and mystery elements tend to gain more attention from fans.
One interesting fnaf animation story is about the haunted animatronics in the pizzeria. They come to life at night and their creepy movements and glitches make for a really spooky tale.
In a particular FNAF story, a group of kids dared each other to spend the night in the abandoned pizzeria. As the night fell, strange noises started. The animatronics that were left there in a dilapidated state began to activate one by one. The kids realized they were in real danger as the animatronics moved towards them with an unnatural speed. They tried to find a way out but the place was like a maze, and they were constantly being chased by the terrifying animatronics.
I'm not aware of any official connection between GTA 5 and FNAF in terms of scary stories. GTA 5 has its own dark and gritty stories, and FNAF has its own spooky lore, but they are from different franchises and not typically combined in that way.