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.
If Jacob and Renesmee get hurt in fanfiction, it can stir up a lot of emotions. Jacob's werewolf friends could be enraged and start a hunt for the culprit. Renesmee, being so young and precious to the Cullens, her injury might make the family reevaluate their security measures. It could also lead to some internal conflict within the family. Some might want to take more drastic actions to protect her while others might be more cautious. And Jacob's relationship with the Cullens could be put to the test. His injury might make the Cullens question if they can really trust him to protect Renesmee.
Unrequited love is a very common theme. Jacob loves Bella deeply, but she has feelings for Edward too. Heartbreak and betrayal also often come up. Sometimes the werewolf pack might seem to betray Jacob's trust in the context of his hurt. Another theme is isolation. Jacob may feel alone in his pain, separated from both the humans and the werewolves at times.
When Jacob and Renesmee are hurt in fanfiction, different characters have different reactions. Carlisle, as the family doctor, would be focused on treating their wounds. Esme would be like a mother hen, fussing over them and trying to make them comfortable. Jacob's wolf pack would be ready to fight. They see Jacob as one of their own and Renesmee as someone they are bound to protect because of Jacob's connection to her. It could lead to a big showdown between the pack and whoever or whatever caused the harm.
Well, there are several 'Twilight Jacob Hurt' fanfics out there. Some can be found on fanfiction.net. You might want to look for tags like 'Jacob Black', 'Hurt' and 'Twilight'. One that stands out is 'Jacob's Agony'. It really captures his pain in a detailed way. It shows how his love for Bella, even when unrequited in some ways, causes him great distress. He has to deal with the werewolf side of him as well as his broken heart, which makes for a very engaging read.
In some fanfictions, Bella might get hurt during a fight with a rogue vampire. For example, she could be caught off - guard while trying to protect someone in Forks. Another scenario could be when she is exploring the woods alone and stumbles upon a dangerous supernatural creature that attacks her.
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.
Jasper would probably be on high alert, using his ability to sense and manipulate emotions to keep the family calm while also being ready to defend against any further attacks. Rosalie might be angry and vengeful, determined to find out who hurt Esme and make them pay. Emmett would be protective, standing guard over Esme while she recovers. The family as a whole would be in a state of shock and concern, their usually peaceful existence disrupted by this event.