Overshot fetlock (“knuckling over”) in the horse

“Knuckling over”, or upright fetlock joints in the horse, is usually caused by a congenital tendon deformity, where the tendons are too short. This causes considerable pain and weakness in otherwise well-conformed horses if corrective surgery is not undertaken soon after birth.

The New Book of the Horse. Charles Richardson, 1913.

Internal organs of the Chicken

Ovis aries - Old Norway Sheep (anomalous, three-horned)

Unlike four-horned sheep (such as the Icelandic and Navajo Churro breeds), three-horned sheep legitimately have more than two horns - in four-horned sheep, the extra pair comes from an offshoot of the first horn buds. As a result, the extra horn locations can deform the skull shape of sheep who develop them, resulting in…”special” sheep.

Thankfully for farmers, most of the time, the extra horn and semi-deformed skull/face has no influence upon brain development, and the sheep are no more dull than average.

Die Säugthiere in Abbildungen nach der Natur. Johann Schreber, 1774.