You've already read about how Beethoven originally wrote the Great Fugue as the finale of his op. 130 quartet, but...

You've already read about how Beethoven originally wrote the Great Fugue as the finale of his op. 130 quartet, but he felt compelled to remove it and write a new finale, later publishing the Fugue separately as his op. 133.

Find another artwork (not necessarily music) that likewise began as one component of a different artwork, but was separated and took on a life of its own equal to (or even surpassing) that of the original artwork.

200 words