You Never Can Tell (1951) starring Dick Powell, Peggy Dow, Joyce Holden (as Goldie, the former racehorse), Charles Drake / From IMDb: A cracker tycoon leaves $6 million to his German Shepherd, King, with his secretary, Ellen, as trustee. King is poisoned, and Ellen inherits but is the prime suspect. King's spirit, which goes to 'Beastatory', thinks he knows who did it and wants the chance to prove it. Enter Rex Shepherd, private eye, who snacks on dog food, and his sweet-talking secretary, Goldie, a former racehorse. But is Rex barking up the wrong tree?
Here's Part 1 on YouTube: http://www.youtube.com/watch?v=hok1JlHVcks