Willis Harold O'Brien is the Father of stop-motion animation. He wasn't the first to experiment with it, in fact Georges Melies, a French filmmaker had used the process to make his fantastic films using mostly cut-outs and to make actors appear and disapper on screen. It was Willis O'Brien who took that process to the level of making inanimate objects appear come to life in a three dimensional realm.
In 1914, O'Brien began animating a series of short subjects set in prehistoric times. He created the films "stars" by covering wooden armatures with clay and then filming then one frame at a time. The first film he created using this process was THE DINOSAUR AND THE MISSING LINK. When Thomas Edison saw the film, he purchased it for theatrical distribution. O'Brien followed this with several others short films including Morpheus Mike, Birth Of A Fliver, R.F.D. 10,000 B.C., Prehistoric Poultry, In The Villians Power, Curious Pets Of Our Ancestors, Mickey and His Goat, Sam Lloyd's Famous Puzzles and Nippy's Nightmare.
Nippy's Nightmare was the first stop-motion film to combine live actors with stop motion caricatures.