In writing your short story about abused animals in a pet store taking their revenge, consider including a variety of animals, developing their characters, and elaborate on how the animals are being abused.

Your characters are abused animals in a pet store, so you will probably want to have a variety of animals: a dog and cat, perhaps, but also a rabbit, a rat, a hamster, a mouse, a bird, or even a ferret. You could give the animals names and develop their characters. Maybe the cat is rather sarcastic and would rather eat the mouse than escape with it but decides to work with the mouse and the others instead (although maybe the smaller animals are a little afraid of the cat anyway). You will also want to describe how these animals are being abused in the pet store. Perhaps they are locked in tiny, dirty cages and are not fed on time or given any attention or love.

Now let's think about your plot. Apparently, these animals escape the pet store, but you will need to decide what leads them to that escape and how they do it. Perhaps someone leaves one of the animal's cages open by mistake, and they decide to take their chances. Since they are clever animals, they work together and open the rest of the cages. Then, they will need to get out of the pet store, so you will have to think about how they do that.

The prompt also talks about the animals taking their revenge. Maybe they mess up the store or chase the owners around or even bring in humane society folks to see the bad conditions and shut the store down. Alternately, you could introduce a human conspirator early in the story who somehow knows what the animals are intending to do and helps them out.

Your setting will be the pet store, so you should describe it vividly. If the animals are abused, the store must not be very clean or pleasant. You could also think about another setting for the end of the story. The animals must go somewhere. Maybe they are all adopted together by the human conspirator, for instance.