Perhaps it's a mysterious figure in the fanfiction world. This unknown character could have some ulterior motives against Jacob and find ways to hurt him, like setting traps for him.
In some fanfictions, Jacob could get hurt while exploring an unknown area. There could be some sort of hidden danger, like a trap set by an enemy. Or perhaps he gets hurt in a natural disaster that occurs while he's in his wolf form and unable to fully protect himself. For example, a sudden landslide or a forest fire.
Bella's actions in the fanfiction might be another cause. Maybe she accidentally says or does something that undermines Jacob's self - esteem. Like if she makes a comment about his werewolf nature in a negative way, it could wound him deeply as it's an important part of who he is.
As I mentioned before, a member of his own wolf pack. In a pack, there can be jealousy and power struggles. If another wolf wants more influence or has a grudge against Jacob, they might be the ones to cause him harm.
Well, in the story of Jacob Black and Leah Clearwater, there is a lot of emotional turmoil. Jacob is constantly torn between his wolf nature and his feelings for Bella. Leah, on the other hand, has to deal with the fact that her ex - Sam imprinted on someone else. This causes her great pain. Jacob and Leah are both part of the werewolf pack, and their interactions are often filled with underlying tension and hurt because of their individual emotional baggage.
Another reaction could be that he tries to hide his pain. Jacob might put on a brave face in front of Bella and the others, but deep down he's really hurting. He could keep his feelings bottled up until it eventually explodes in a moment of weakness or when pushed too far.
In these fanfictions, Jacob often retains his loyalty, which is a big part of his character. He's extremely loyal to his pack and to the person he loves. For example, in many stories, he'll go to great lengths to protect his love interest from any danger, whether it's from other supernatural beings or just regular human problems.
I'm not sure if there is a specific 'Jacob gets Renesmee pregnant' fanfiction. Fanfiction can cover a vast range of ideas, but this is a rather controversial and unusual plot. It might exist in some corners of the fanfiction world, but it also goes against some of the original concepts in the source material. You would need to search through various fanfiction platforms like Archive of Our Own or FanFiction.net to find out for sure.
There might be some extremely niche fanfiction out there with this concept, but it goes against the established lore of the 'Twilight' universe in many ways. Vampires in the 'Twilight' world are undead and have different biological functions compared to humans, and werewolves also have their own unique biology. So while fanfiction can be creative, this would be a very out - of - the - box and perhaps not very common plot.