It's a Dog's Life (1984)

While Jessica attends a hunting party, someone tampers in the stables with the normally quiet horse of the rich host, Denton Langley, which suddenly goes wild and causes him to have a fatal fall, yet the drug test Jessica suggests is negative. The will, on videotape, gives his shocked relatives and estate staff peanuts while $15,000,000 goes to Denton's pet dog, Teddy, who is shortly after found drugged after biting old local Isaiah Potts. Family lawyer Marcus Boswell is challenged by the human heirs' lawyer on account of the dog's mental competence. Then Trish Langley is found killed by the security gate, the button which operates it from security guard Barnes' closed room has Teddy's paw print on it. He must have been trained to press it. Jessica works out precisely what ties in, how, and what doesn't.