During the 1920s, before the herds were managed by various agencies, many conformation faults were found—the effects of uncontrolled inbreeding. Misshapen legs, narrow chests, poor bone and a lack of substance plagued the breed, with many stunted animals not growing above . This was partially due to the limited and poor-quality feed found on the islands, although this harsh habitat also allowed only the hardiest and most adaptable ponies to survive. Welsh and Shetland pony blood was added to upgrade the stock, and horses with pinto coloring were introduced to give the herd its common distinctive patterns and contribute to the more horse-like phenotype of the breed. Twenty Mustangs owned by the Bureau of Land Management were introduced in 1939. Arabian blood was added in the hopes of adding refinement and height to the breed, as well as increasing the length of their legs. Arabian stallions were used at two different points within the breed history: one was released with the herd, but did not survive, while another was bred to mares that had been removed from the island for breeding and then returned once in foal. The Chincoteague pony has a similar history to the Shackleford Banker Horse, which comes from the Shackleford Banks off the coast of North Carolina. However, the Shackleford is a more isolated population, with no outside blood added to the island herd.