In the recent Victor Frankenstein with James McAvoy in the title role, we have seen Daniel
So no, there isn't a single trace of an Igor in Mary Shelley's novel, nor is there any character who could have inspired the deformed valet pulling switches in the popular imagery. No Igor either in any of the theater plays that have been staged throughout the 19th century. No Igor in the first film version of Frankenstein in 1910.
Could we date the creation of this character with the release of James Whale's Frankenstein in 1931? Everyone remembers Boris Karloff being persecuted by a hunchback holding a torch, played by Dwight Frye... Unfortunately, the guy isn't called Igor, but Fritz. Damn. In fact, Dwight Frye appeared in most of Universal Frankenstein films, as a different character each time, but he never played an Igor. J. Carrol Naish plays another hunchback in House of Frankenstein, but his character is called Daniel!
The first time a Frankenstein met an Ygor (with an Y) was in 1939 Son of Frankenstein, the third entry in the Universal saga. Played by none other than Bela Lugosi, this Ygor introduces himself to Wolf von Frankenstein (Basil Rathbone), and says he used to be his father's assistant – which is rather dubious since we haven't seen him in previous movies. Besides, he's not a hunchback at all, he simply has a broken neck due to his attempted hanging by the townspeople. Lugosi's Ygor returne in the sequel Ghost of Frankenstein, then lived on as the Monster's brain in Frankenstein meets the Wolf Man.
There was no character called Igor in the Frankenstein movies produced by Hammer Films, and the only occurrence that can be spotted in the 60s is the western schlock Jesse James meets Frankenstein's Daughter, where said daughter renames her creature Igor for unknown reasons.
By the time Mel Brooks made his Young Frankenstein in 1973, the hunchback assistant Igor was a given in most people's minds, probably because of a confusion between Dwight Frye and Bela Lugosi's characters. However, Young Frankenstein's Igor played by Marty Feldman marks his real screen debut. Innumerable other Igors would follow, in movies such as Mistress Frankenstein, The Nightmare before Christmas, Frankenstein Italian Style, Return of the Killer Tomatoes, Van Helsing, Crazy Dracula Spring Break Weekend, Christmas at Draculas and even... wait for it... Daughter of Werebitch Meets Skankenstein.
In 2008, there was even a feature animated film starring Igor, with the voice talents of John Cusack as Igor and John Cleese as Dr Glickenstein. In this (dark) comedy, Igors are a race of slaves doomed to assist crazy scientists in their experiments. But our hero has higher ambitions!
There is no character called Igor in Sherlock Holmes vs Frankenstein.