He wasn't always cast as a monster. On House of Frankenstein he played a scientist. An evil one, sure, but Glen Strange played Frankenstein's monster in that movie.
Karloff's big break came from playing the monster in the 1931 Frankenstein, and since he was famous for his role in a Horror movie he continued to be cast in horror movies.
The cast of The Son of Frankenstein - 1966 includes: Lionel Atwill as Police Official Donnie Dunagan as Peter von Frankenstein Emma Dunn as Amelia, housekeeper Boris Karloff as The Monster Bela Lugosi as The Hunchback Edgar Norton as Benson, butler Basil Rathbone as Frankenstein
If you mean the version directed by Roger Corman in the 1960's , they were Boris Karloff, Peter Lorre, Hazel Court, and Jack Nicholson.
Monster-in-Law
The cast of The Prisoner - 1923 includes: Mario Carillo as Count Sallonica George Cowl as Lord Bob June Elvidge as Lady Francis Bertram Grassby as Prince Ugo Ravorelli Hayford Hobbs as Duke Laselli Boris Karloff as Prince Kapolski Lillian Langdon as Mrs. Garrison Eileen Percy as Dorothy Garrison Esther Ralston as Marie Herbert Rawlinson as Philip Quentin Gertrude Short as Lady Jane Bert Sprotte as Courant Lincoln Stedman as Dickey Savage
The cast of My House - 2011 includes: Justin Beecher as The Monster Matt Seaton as Man
The cast of The Bounty - 2011 includes: Daniel Cason as Monster Lindsay Hocking as Dr. Boris Karloff
The cast of Boris Karloff and Bela Lugosi - 2002 includes: Bela Lugosi as himself
Frankenstein. Karloff acted in eighty movies before being found by James Whale and cast in Frankenstein (1931). Karloff's role as Frankenstein's monster was physically demanding – it necessitated a bulky costume with four-inch platform boots – but the costume and extensive makeup produced a lasting image.
Black Sabbath (1963)
The cast of Extra Weird - 2003 includes: Boris Karloff as himself
The cast of The Juggler of Our Lady - 1958 includes: Boris Karloff as Narrator
The cast of The Son of Frankenstein - 1966 includes: Lionel Atwill as Police Official Donnie Dunagan as Peter von Frankenstein Emma Dunn as Amelia, housekeeper Boris Karloff as The Monster Bela Lugosi as The Hunchback Edgar Norton as Benson, butler Basil Rathbone as Frankenstein
The cast of Forbidden Cargo - 1925 includes: Robert Ellis as Jerry Burke Boris Karloff as Pietro Castillano
The cast of The Hellion - 1924 includes: William Berke as The Father Aline Goodwin as The Daughter Boris Karloff as The Outlaw Marin Sais as The Hellion
The cast of Sous les verrous - 1931 includes: Oliver Hardy Boris Karloff as Le Tigre Stan Laurel
The cast of Corridor Gossip - 2007 includes: Robert Day as himself Boris Karloff as Dr. Thomas Bolton Francis Matthews as himself
The cast of Parisian Nights - 1925 includes: Gaston Glass as Jacques Elaine Hammerstein as Adele Boris Karloff as Pierre Lou Tellegen as Jean