The most common failure in fable writing is the moral that arrives like a lecture. If the last line says "and so the fox learned not to be greedy," the reader has been told what to think. A better fable lets the consequence speak. The fox loses the cheese, and the reader draws the conclusion without any help.
Characters are chosen to carry meaning. A fox suggests cunning and a lion suggests power, but those associations are well worn, so we sometimes pick less familiar animals to keep the story fresh. The animal should fit the lesson, and the lesson should fit the animal.
Economy matters. Every sentence earns its place, and the story usually moves in three beats: a situation, a choice and a consequence. Repetition can be a strength, since children enjoy a pattern they can predict, but the third time must differ from the first two.
Sound matters too. A fable often gets read aloud, so we test the rhythm, the sound of names and the pause before the final line. A good ending is short and slightly surprising.
Finally, we check the tone. Humor is welcome, cruelty rarely is, and the lesson should fit the values you want to pass on. If your fable will meet young listeners, we avoid frightening imagery unless the story truly needs it.