The cliché has its origins in the character of Ygor, played by Bela Lugosi, in the Universal Studios horror movies Son of Frankenstein and Ghost of Frankenstein; it also owes something to the hunchbacked lab assistant in the first film of the series, whose name was Fritz. The archetypal Igor, however, is probably the character of that name played by Marty Feldman in Young Frankenstein, Mel Brooks's parody of Universal's Frankenstein movies.
In Terry Pratchett's humorous fantasy novels, the Uberwald region of the Discworld (that is, the region of the Discworld noted for resembling a collection of horror movie clichés) is home to an extended family of hunch-backed lab assistants with speech disabilities, every single one of whom is named Igor.
The "Albino" character in The Princess Bride seems to be built on the Igor archetype.
In general, an Igor is any flunky, patsy, minion or bidding-doer in a fantasy or science fiction work--and the more disfigured the better.