1) A span of horses consists of two of nearly the same color, and otherwise nearly alike, which are usually harnessed side by side. The word signifies properly the same as yoke, when applied to horned cattle, for buckling or fastening together. But in America, span always implies resemblance in color at least; it being an object of ambition with gentlemen, and with teamsters, to unite two horses abreast that are alike.--Webster. This use of the word is not mentioned in any of the English dictionaries or glossaries.
TO SPAN
2) To agree in color, or in color and size; as, 'The horses span well.' New England.--Webster.