"Pigs Is Pigs" is a story written by Ellis Parker Butler. First published as a short story in American Illustrated Magazine in September 1905, "Pigs Is Pigs" went on to dozens of printings as a book and in anthologies over the next several decades.
Railway agent Mike Flannery wants to charge the livestock rate for a shipment of two guinea pigs and refuses to accept the lower pet rate, saying "pigs is pigs". Flannery believes that the "guinea" is an indication of the pigs' national origin. He argues that they should bear the higher freight charge of 30¢ for livestock, rather than the lower 25¢ for domestic pets. In support of this, he submits that if they were "dago pigs" or "paddy pigs", there would be no question of the animals' status.
Because the customer refuses to accept delivery, Flannery is forced to feed and house what he now calls the "dago pigs" in his office, until he receives permission from his superiors to return the pigs to the company warehouse. By this time, the guinea pigs have reproduced exponentially in Flannery's station house. After returning all the descendants, Flannery resolves to charge the lower rate for any future livestock.
The story was credited by Robert A. Heinlein as possibly the origin of the flat cats in his novel The Rolling Stones, since he may have read it or heard it as a child, but due to the intervening time he could not be sure. The flat cats in turn may have been David Gerrold's unconscious inspiration for the Star Trek universe's "tribbles".
The story was made into movie form several times, including the Walt Disney cartoon in 1954 that was nominated for an Academy Award for Animated Short Film.
The 1937 Merrie Melodies cartoon, directed by Friz Freleng, is not an adaptation of the Butler story, though it shares its title.