THE BEAST WITHIN   (1981) Suspense/Horror Like many adolescents, a Mississippi teenager finds his body and his appetites growing beyond human proportions--only this time, it's a horror movie. The source of his urges comes from the day of his conception, the result of his mother's rape by a monster, and now the boy wants to continue the tradition of his absent father. CAST Ronny Cox, Bibi Besch, Paul Clemens, R.G. Armstrong, Don Gordon DIRECTOR Philippe Mora