Mail Dog (1947) is a classic animated short film by Walt Disney Productions that features Pluto, who plays the role of a mail dog and is tasked to deliver mail and is accompanied by a playful rabbit.
When a pilot has to turn back due to a severe storm, he drops the mail at a remote outpost where it can be delivered by dogsled. The falling mail pouch lands on Pluto, and he sets out to deliver it.
An explosion, a conspiracy, and an unlikely duo on a mission to expose the truth.