Some people have said that the Grimm Brothers wrote the fairy tales, and this is not actually true. They collected the tales and re-wrote them in their own style, but they didn't write them. Most were around many years (even hundreds of years) before the Brothers were born.
One person has posted several times on the show's website that Sheriff Graham was supposed to be Robin Hood, but that the producers could not obtain the rights to Robin Hood. This is false information. There are no "rights" to be owned. The tale of Robin Hood is a folk tale, hundreds of years old and no one knows for sure whether there was such a person or who he was. It's possible the writers/producers initially wanted Graham to be Robin Hood then changed their minds, perhaps to bring in Robin Hood later or they didn't like the way the character was fitting into the story.