It could be that the hamster didn't have a proper diet or enough water. Poor nutrition can cause health problems that lead to death. Also, if its living environment was unclean or too stressful, that might have affected its well-being.
Maybe it got sick and didn't recover. Sometimes hamsters catch infections that are hard to treat.
One possible theme could be friendship. For example, the members of One Direction bond over taking care of a hamster together.
In the fanfiction, it might be that there was a powerful and unexpected dark magic curse placed on the Potter family long before Voldemort's attack. This curse was designed to target the youngest Potter, and despite the efforts of his parents, it couldn't be stopped, causing Harry to die as a baby.
One possible reason could be a dark magic curse. In many fanfictions, an ancient or powerful dark magic spell might be placed on Harry, and despite all his skills and the help of his friends, he couldn't overcome it. This could be something that was secretly cast on him during his early years, perhaps related to his connection with Voldemort.
Maybe he was killed by Voldemort in a more sudden and unexpected way when he was a child. Since Voldemort was always after him, it's possible that he managed to get to Harry earlier in this fanfiction.
Maybe there was a severe shortage of food in the area where the kid lived. Poverty could also be a factor, making it hard for the family to afford enough food.
One reason could be an unskilled masseur. If they don't know the right techniques, they might cause pain or discomfort. For example, using too much pressure or the wrong kind of strokes.
In some fanfictions, she could die sacrificing herself for her friends. Twilight is extremely loyal to her friends in Ponyville. So, if there was a situation where her friends' lives were in danger, like from a powerful villain, she might choose to give up her own life to protect them. This would be a very heroic and in - character way for her to die.
One possible reason could be a lack of communication. If there were misunderstandings that weren't cleared up, it might lead to abandonment.
One reason could be for a fun family event like a themed party. Maybe it's a 'gender - bender' party and dressing your son as a girl is part of the concept. Another reason might be for an art project or a photo that has a creative concept behind it.
One possible reason could be that it faced some dangerous creatures in the time they traveled to. For example, if they went to a prehistoric time, dinosaurs or other large predators might have attacked the dog.