The Incredible Two-Headed Transplant (1971)

Mad scientist Dr. Roger Girard (Bruce Dern) is experimenting with transplanting second heads onto farm animals. When an out-of-control maniac, Cass (Albert Cole), stumbles upon Girard's place, he kills the landlord, injures his dim-witted son, Danny (John Bloom), and seriously hurts himself, as well. Seizing the opportunity, Girard grafts Cass's head onto Danny's body in an effort to save them both. The experiment is successful, but the creation goes mad and wreaks havoc on the countryside.