A Philadelphia Times article from 1887 wrote that the locals believed that the horses aboard the wrecked ship were Shetlands. The writer described the ponies at the 1887 penning as, "In size the Chincoteague pony approximates that of the Shetland. The hair on their bodies is thick and shaggy and their manes and tails are long and glossy." In 1916 The Country Gentleman wrote that several landowners had turned Shetlands out on the island with the idea of creating smaller stock.

The Baltimore Sun ad in 1892

A herd of Shetlands owned by Edward P. Timmons was kept on Pope Island in the early 1900s. Some of his Shetlands were sold at a sale during Pony Penning 1912. A Daily Banner article in 1920 wrote that a Shetland owned by Timmons was brought to the island and and made the ponies smaller but more hearty. Timmons had sold his ponies during Pony Penning and in years past and liquidated his herd during Pony Penning 1914. Pope Island is alongside Assateague and is within swimming distance so it's possible some of his ponies were incorporated into the Assateague herds.

Once Upon an Island by Kirk Mariner states that Clarence Beebe imported a Shetland for breeding from which the many pintos found today descend. Ronald Keiper wrote in his book The Assateague Ponies that pinto was introduced in the 1920's. An Associated Press article from July 1925 states, "The strain is not so pure now as it was in former years. The Shetland pony together with other breeds has been introduced and the effect has wrought variety." Clarence Beebe was advertising spotted ponies for sale in 1920. The Horse and Pony Book, published in 1954, said "long ago a piebald Shetland stallion was taken to live with the wild ponies,. A decade later in 1963 America's Wild Horses said "a little Shetland stallion was turned out among them and his characteristics have been seen ever since".

A Chincoteague Pony management plan from 2014 stated that "A wide variety of breeds such as Morgan, Welsh, Shetland, Arabian, and Mustangs were placed in the Chincoteague pony herd to increase genetic diversity and vigor among the present stock".