The suit used in the film was a bog-standard gorilla suit with hair and wigs added to it. You'd never tell watching the movie however as clever camerawork, focus, and darkness often make the Fouke Monster an eerie silhouette against the dark swamps.
The screams the monster makes, which can best be described as a echoing shriek and bellow at once that do not sound entirely inhuman, are positively bone chilling.
