The Thief was introduced as a character class in Dungeons & Dragons through the Greyhawk supplement in 1975. Ideas are built on other ideas but you really can’t trace the D&D Thief class back to perhaps the most famous literary thief, Robin Hood. Sherwood’s Prince of Thieves would be a Ranger in D&D, not a Thief. The inspiration for the original D&D Thief class must have come from somewhere else. Could it be from thieves introduced in the 1930s, Bilbo Baggins (1937), The Gray Mouser (1936) and Conan the Barbarian (1932)? Yes, of course, that’s plain to see. And certainly there are much older influences in myths and characters throughout humankind’s literary history. But D&D was initially fashioned together in the early 1970s so the most powerful influences were likely the recent ones, especially Jack Vance’s Cugel (1965) and Roger Zelazny’s Jack of Shadows (1971).
From a fanzine in 1974:
A NEW CHARACTER TYPE FOR DUNGEONS & DRAGONS: THE THIEF!
by Gary Gygax
Recently I received a telephone call from Gary Schweitzer who hales from sunny California. It isn’t all that sunny out there, however, for are many dungeon expeditions regularly being led beneath the grim pile of the castles which are scattered throughout that land. Anyway, during the course of our conversation he mentioned that his group was developing a new class of character – thieves. Gary gave me a few details of how they were considering this character type, and from these I have constructed tentative rules for the class. These rules have not be tested and should be treated accordingly.
Clearly, the Thief class was not Gary Gygax’s original idea. However, Gygax charted the Thief’s evolution in D&D and has recognized both Zelazny and Vance as inspirations. Zelazny’s Jack of Shadows, though important to the Thief’s development, was an homage to the characters and worlds created by Vance. Therefore, I consider Vance’s work to be the primary source material and as I read stories about his own thief, Cugel, I will return to comment on this indelible influence.