True; but most stories are taken from somewhere else. Original thought is incredibly rare.

For example, much of Tolkien's LOTR was from old anglo-Saxon legends and myths. In fact the 'cursed ring' idea came from a very real 'cursed ring' (called the Silvianus Ring) a ring belonging to a british-Roman, and subsequently stolen. The owner, Silvianus, crrated a curse (written on a stone tablet, which still exists) to bring poor health and madness to whoever wears it, until it is returned.

Lots of parts of Tolkien's works can be traced to old myths and legends (primarily anglo-saxon as mentioned) he reshaped them and stitched them all together.

Shelley was ultimately 'set homework' by Byron via his challenge, and she wanted to win.. she needed a story quickly.

Another guest at the party with Byron was JW Polidori.. who wrote the 'Vampyre'. This was once again a pre-existing story re-hashed (as was dracula etc etc).

Shelley comes into a lot of criticism (perhaps because she's a female?) - the only thing that she is incorrectly credited with, is the first horror/gothic novel. Gothic/horror stories had existed for 100s of years prior to Shelley's book - there are countless examples, including the book that Byron read to them all that evening.