Pigs Is Pigs (1954)

"Based on the story by Ellis Parker Butler"

Flannery, a railway agent does everything by the book. He gets into a scrape with a customer, McMorehouse, who wants to pay 44 cents freight for two guinea pigs which he considers pets. Flannery, however, considers them pigs (freight 48 cents), a decision he begins to regret when the animals begin to reproduce.