The Creeper character isn't in the short stories or novels (I've read them all), although he is possibly based on the character Beppo from The Six Napoleons. The Creeper appears in The Pearl of Death - A Basil Rathbone/Nigel Bruce Holmes tale from 1944 based on The Six Napoleons.

Also, the Creeper returned in a couple of unrelated films - House of Horrors (1946) and Brute Man (1946), before the actor Rondo Hatton died.

I like the Rathbone Holmes films as they are atmospheric and pacy (the later Robert Downey Jr/Jude Law pictures very much take their cue from the Rathbone era in that respect). They play fast and loose with the source material, sometimes adding elements that actually improve on the original text. For example, the whole broken china thing in The Pearl of Death is a very nifty addition to The Six Napoleons story.

On the downside, they made Watson rather a buffoon - a stereotype that sadly stuck for some time, until the faithful (and as far as I'm concerned definitive) TV series in the 1980s, with Jeremy Brett as Holmes. The Rathbone era also churned out Moriarty on too much of a regular basis, whereas Conan Doyle used him far more sparingly (to largely indifferent effect in The Valley of Fear and to great effect in the classic The Final Problem).