E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A region of calms between 30° and 35° North; so called becuase ships laden with horses bound to America or the West Indies were often obliged to lighten their freight by casting the horses overboard when calmbound in these latitudes.
Nothing could have been more delightful than our run into the horse latitudes. Gales and dead calms, terrible thunderstorms and breezes, fair one hour and foul the next, are the characteristics of these parallels. Numbers of horses were exported from the mother country, and it was reckoned that more of the animals died in these latitudes than in all the rest of the passage.Clark Russell: Lady Maud, vol. i. chap. vii. p. 186.