Andrew Hearst of Panopticist says:
Created anonymously by a group of professional animators in about 1929, the silent short Eveready Harton in Buried Treasure is a gleeful exploration of the penetrative arts.
The four-and-a-half-minute short follows the travails of the uncomfortably well-endowed title character as he wanders a barren landscape in search of satisfaction. Along the way, he encounters a self-pleasuring maiden, various sexually aroused animals, a surprised husband, and a cow-humping farmer, whom Harton challenges to a duel. A penis duel.
Eveready Harton in Buried Treasure is one of the earliest examples of an animated porn film. According to its Wikipedia page, several famous animators supposedly made the short for a private party in honor of the pioneering animator Winsor McCay, whose work greatly influenced Walt Disney and is still held in high esteem by Maurice Sendak, Chris Ware, and other luminaries.
Video is NSFW Link