Maybe the fox gets trapped in a cage by some cruel humans who want to sell it to a zoo or something, and it misses its freedom in the wild. That would be a sad plot.
Maybe the main plot is about a family in Roblox. A group of players formed a virtual family, and they were really happy. But then, some members started to have real - life issues and had to leave the game. The remaining family members in Roblox felt abandoned and sad, as their virtual family was falling apart.
A possible main plot is that a new and powerful adversary emerges. This adversary uses mind - control technology on the Star Fox team. Under the control, they are made to fight against their own allies, which is not only a physical defeat but also a huge humiliation as they are being used against their will. When they break free, they have to deal with the shame of what they've done and regain their reputation.
Maybe the little rascals are bullied by some older kids. They feel powerless and scared, and this is the source of the sadness in their story. They might try to stand up for themselves but end up getting into more trouble.
One possible main plot is that the shiny Eevee had a dream of evolving into a certain form, like Sylveon. However, it faced so many obstacles in its life that it couldn't gather the necessary items or experiences to evolve. It watched other Eevee around it evolve into their desired forms while it remained stuck, which made it feel sad and inferior.
Perhaps the main plot of the sad boxer dog story is that the boxer dog was born with a physical defect that made it different from other dogs. It was shunned by other animals at the park and sometimes even by people. This constant rejection made the dog very sad, and it was always looking for a place where it could be accepted and loved.
The main plot might be that Pat and Jen were in a romantic relationship. Jen had always dreamed of getting married and having a family, but Pat was afraid of commitment. After years of being together, Pat finally broke up with Jen, leaving Jen feeling rejected and heartbroken. Pat also felt sad because he did lose someone he cared about, but his fear got the better of him.
Perhaps Celly had a pet that she loved very much. One day, the pet ran away and never came back. Celly searched everywhere for it, asked the neighbors, put up posters, but still no sign of the pet. She missed the pet's company, the way it would greet her when she came home, and this made her really sad.
It could be that John Doe was part of a Roblox group that he really liked. He was very active in it and contributed a lot. But then, for some unjust reason, he was kicked out of the group. This sudden expulsion made him feel sad and betrayed, which could be the main plot of the sad story.
Well, it could be that a fox from the wild develops a forbidden love with a domesticated fox, and they have to overcome human interference and their own instincts to make it work. There could also be rival foxes or other predators creating obstacles along the way.
The 'a sad sim story' might revolve around a Sim's unrequited love. The Sim falls deeply in love with another Sim who doesn't feel the same way. This leads to a lot of heartache and sadness for the first Sim. They might try everything to win the other's heart but end up getting rejected repeatedly, which makes them withdraw into themselves and become a sadder version of themselves.