Rusty might have been separated from his family. For example, if his family moved and couldn't take him along, he would be left in a strange place, not understanding why he was alone. This would be a really sad situation for Rusty.
I'm not sure specifically which 'rusty the dog sad story' you mean. There could be many stories about a dog named Rusty that are sad. Maybe Rusty was abandoned by his owner and had to fend for himself on the cold streets, looking for food and shelter all alone.
Once there was a dog named Max. He was born deformed with a crooked leg. Other dogs would often bully him at the park. His owner, a kind old lady, tried her best to protect him. But Max was always sad, feeling different from the others. One day, while trying to run like the normal dogs, he fell hard and hurt his already weak leg. It was a really sad sight as he whimpered in pain.
There was an animated dog in a movie. His name was Buddy. Buddy was always so happy, running around the park with his owner, a kind old man. But one day, the old man got sick and passed away. Buddy didn't know what to do. He was left alone in the big, empty house. He howled at the moon every night, his cries filled with sorrow. He stopped eating and just lay in his corner, his eyes dull and sad. Eventually, the neighbors had to find a new home for him, but Buddy was never the same again.
There was a dog called Buddy. Buddy's owner passed away suddenly. Buddy didn't understand what had happened. He would sit by the front door every day, waiting for his owner to come home like he always did. He stopped eating well and lost his playful spirit. The new people who moved into the house tried to befriend him, but Buddy just seemed so lost in his sadness for his old master.
A sad dog perspective story could be like this. There was a little dog called Buddy. Buddy used to live in a warm and loving home. But one day, his family moved away and left him with a neighbor who didn't really care for him. Buddy missed his old bed, his toys, and most of all, his family. He would look out of the window every day, waiting for them to return. His eyes were always full of sadness, and he barked less and less as time went by, because he was losing hope.
The sad story could be about an abandoned old dog. The dog that found it saw the poor old thing with matted fur, skinny and weak, left behind by its owners who moved away or just didn't want it anymore. It's a common and really sad situation for many dogs.
Once there was a sad dog in a cartoon. The dog was left alone in an old house when his family moved away. He waited day after day for them to come back, sitting by the window with a forlorn look. His little tail that used to wag so happily now just hung limply. Every time he heard a car pass by, he hoped it was his family. But as days turned into weeks, his hope started to fade and he became more and more sad.