Francis (1950) on Netflix

During World War II, a junior American Army officer, Lt. Peter Stirling, gets sent to the psychiatric ward whenever he insists that an Army mule named Francis speaks to him.

The truthful soldier Stirling didn't know how to lie about his source of information, the talking army Mule, Francis, so he was treated as a lunatic and led to one after another hilarious situations, where the mule was the only one that appeared in his right mind. In the process of all this, the mule assisted in uncovering a spy, Maureen, who pretended to be lost among the jungles.

—Winston Song