Chestnut (ee), also known as sorrel and red, is one of the foundation colors. Ponies are black, chestnut, bay, or brown with other colors diluting/modifying the base colors. Extension and agouti form the basis of all equine color. Extension determines whether or not a pony has black or not and agouti determines where it will go. Recessive extension creates the chestnut or red color. Chestnuts carry agouti but without the dominant form of extension producing black agouti has no effect. The agouti status of chestnuts cannot be determined visually and must be tested for.
Chestnut is a recessive so a pony must have two copies of chestnut in order to be chestnut. Two chestnut ponies cannot produce anything but chestnut.
Some chestnuts have manes and tails that are darker than their body color. These chestnuts are referred to as tostado chestnuts.
Chestnut based ponies will generally have more white than black based ponies because black is a known suppressor of white. Chestnut based ponies can have minimal white, however it will be less common than in black based ponies.
Chestnut is likely an original color in Chincoteagues as it is widespread and the breed was originally all solid dark colors. A 1891 article in the New York City newspaper The Sun stated that the ponies "are most frequently black, gray, sorrel, or dun [buckskin]." Leonard D. Sale wrote in 1896 in The Horse Review of Chicago that, "The prevailing colors are bay, brown, chestnut and light sorrel. A 1923 St. Petersburg Times article described the ponies as "bay, gray, dun [buckskin], black, and sorrel".