Generally referred to as an unimproved or heirloom breed the Jacob is descended from an ancient Old World breed of sheep although its exact origins remain unclear. Spotted polycerate sheep were documented in England by the mid-17th century and were widespread a century later. Unlike most other old world breeds, the Jacobs of North America have not undergone extensive cross-breeding and selective breeding; their body habitus resembles that of a goat. Relative to their American counterparts, British Jacobs tend to be larger and heavier, and have lost many of their original characteristics through artificial selection.The Jacob is a small, multi-horned, piebald sheep that resembles a goat in its conformation. However, it is not the only breed that can produce polycerate or piebald offspring. Other polycerate breeds include the Hebridean, Icelandic, Manx Loaghtan, and the Navajo-Churro, and other piebald breeds include the Finnsheep and the West African Dwarf.
Mature rams weigh about 120 to 180 pounds while ewes weigh about 80 to 120 pounds The body frame is long, with a straight back and a rump that slopes toward the base of the tail. The rams have short scrotums free of wool which hold the testicles closer to the body than those of modern breeds, while the ewes have small udders free of wool that are also held closer to the body than those of modern breeds. The head is slender and triangular and clear of wool forward of the horns and on the cheeks. The tail is long and woolly, extending almost to the hock if it has not been docked. Jacob owners do not usually dock the tail completely, even for market sheep but instead leave several inches to cover the anus and vulva. The legs are medium-length, slender, free of wool below the knees, and preferably white with or without colored patches. The hooves are black or striped It is not unusual for Jacobs to be cow-hocked. They provide a lean carcass with little external fat, with a high yield of meat compared to more improved breeds.