Fantasy role playing games, be they pen & paper or electronic, are filled with a vast variety of original ideas and concepts… a great many of them stolen.
One major source of theft is mythology, Greek, Norse, Celtic or (rarely) other, but game designers often find their ideas outside the pages of Bullfinch - inspiration is commonly found in the works of Tolkien, Robert E. Howard, Tolkien, Jack Vance, Tolkien and others. The belief being, if you have to steal, steal from the best, or at least from whoever the guy in the previous game stole from.
Let’s try to trace the origins of fantasy RPG (or science fiction, if you like) rip-offs. Here are a few examples (all from D&D, but don’t let that limit you):
Vorpal Sword - everyone’s favorite decapitator. From Lewis Carrol’s Jabberwocky.
Trolls - trolls are common throught northern myth and its derivatives, but the D&D regenerating troll was invented by Pol Anderson in Three Hearts and Three Lions.
Gythyanki - emaciated mind-blasters. A race of beings by that name is mentioned by George R.R. Martin in his “known Universe” books from the early 1970’s (Tuff Voyaging, Nightflyers, Dying of the Light). They’re described as a “slave race” with psychic powers.
Elven Boots - elven cloaks are from Tolkien, obviously, but the matching footware, which makes no sound and leaves no footprints, is from Dilvish the Damned by Roger Zelazny.
So, any more to add? Ever wondered where the hell they came up with a certain beast or artifact, and would like to ask around? Now’s your chance