Devil's Partner (1961)

Starring Ed Nelson, Edgar Buchanan, Jean Allison, Richard Crane, Spencer Carlisle
Directed by Charles R. Rondeau
(actor & director credits courtesy IMDB.com)

A bitter old hermit sells his soul to the devil and in return is given youth, which he uses to pass himself off as his own nephew, and the power to possess the bodies of animals.

Although low budget and possessing little in the way of special effects, this is a compelling little horror picture, notable for its cast, and a terrific unheralded music score from Ronald Stein that's among my favorites of his.  Nelson stars in both of the hermit's guises and gives a good performance, making his swift convincing of the townspeople that the kindly nephew is nothing like the cruel old man utterly believable. I liked him and all the cast- reliable character actor Buchanan is also most welcome as the town doctor, and Byron Foulger is almost unrecognizable as a dirty vagrant. As a horror film, I found the movie effective even with much of the violence occurring off camera, and a good bit of humor also featured in the script.  The animal possession sequences are nothing special, and perhaps could have been better with more money, but regardless I think this is fun viewing for any old-time horror fan.