Aesop has never writen a fable. He used to tell them to the people, orally, who passed it from one generation to another, through the oral tradition.
It is almost impossible to set a date for the creations of these fables, especially because many of them come from ancient Asia and Africa. Aesop might have heard them and adopted the stories to his own style. even his existence is not proven. The history tells that he was a freed slave, but there is nothing to prove any story about him.
After many years of his death, his fables started being printed in plates or fabric, and kept being passed through orality, until it comes to our time, and the fables being attributed to Aesop as we know.
This explains why some of the fables appeared on Egyptian papyrus a thousand years before his birth.