Source: Warren, keep Watching the skies
The monster for this film, "IT", was designed and built by famous 1950s monster maker Paul Blaisdell ("It Conquered the World," "Invasion of the Saucer Men," "The She-Creature," etc.) For most of his AIP movies, Blaisdell had worn the monster suits himself, so designed them for his own slight body type. He did not wear the IT suit for this United Artists film - it was worn by former cowboy and stuntman Ray "Crash" Corrigan, a very large man. Corrigan's head was so much bigger than Blaisdell's, his chin stuck out of the mouth opening of the mask. There was no time or money to make a new mask, so Corrigan's chin simply served as the monster's tongue!