Dude, just about all of Disney's big and famous movies were based on other works and stories.
The Lion King
The Little Mermaid
Just to name a few.
Actually, Disney claimed The Lion King to be their first original story, despite it being a remake of Bambi.
The first original Disney story is actually Land and the Tramp. Due to a bit of hand waving, though, credit was given to Ward Greene. The movie started as a completely in house idea from Joe Grant based on his own Springer Spaniel named Lady where they played up the dog vs. new baby aspect.
They toyed with the concept of Lady and the baby being the basis of the story for a few years until it was shelved. Later, Walt then read a short story in Cosmopolitan (it was a much more literary magazine back then) written by Ward Greene called "Happy Dan, The Cynical Dog". Walt liked Happy Dan and he became the prototype for Tramp. Disney then tossed the idea back to his team for development. It was on and off for another a decade going through several iterations before we got the movie we ended up with.
Ward Greene was hired by Disney to write the novelization and he got the "based on" credit. But the truth is that Joe Grant should get most of the credit as it was his original idea and story ideas that later Disney story-borders managed to cobble together into a complete movie.