It's no secret. Jean de la Fontaine was an Academician (as in, the French Academy) around the time of the ancients vs moderns quarrel. As a member of the former, la Fontaine believed everything good had already been written and all they could do was retell old stories.
He himself claimed to have based his fables on the writing of, among others, Aesop.
And he was the bane of fables to memorize and recite when I was a kid. Always struck me later on with ‘la cigale et la fourmi’ was always praised as a good lesson but that it was a bit cruel. I always preferred maitre corbeau avec son fromage.