Why are the horse latitudes called by that name?
The Horse Latitudes, also know as the Doldrums, is "two belts of latitudes located over the oceans at about 30° to 35° north and south of the equator" where winds are generally more calm than in higher latitudes. Sailing ships tried to avoid the Horse Latitudes when they could, due to fear of being becalmed (stuck) there and running out of drinking water or food.
The origin of the term is subject to debate; the answer above, that sailors threw their horses overboard to lighten the ship, has been deemed by scholars as unlikely to be correct. The majority of trading ships would not have carried horses to begin with, as they did not fare well and created a lot of work for the crew. Additionally, fresh meat would have been at a premium at sea, and access to it might have made a life-or-death difference for a becalmed crew; why would they throw a source of food overboard?
The more likely explanation appears in a natural history text published in 1535, which says the term derives from El Golfo de las Yeguas, which translates to "The Mares’ Sea". The sailors called it this because in the 1500's there was active shipping of horses, particularly brood mares, from Spain to the Canary Islands, and many of the horses died during the transit of this area.
Between 30 & 35 degrees north and 30 & 35 degrees south of the equator lies the region known as the horse latitudes or the subtropical high. This region of subsidising dry air and high pressure results in weak winds. Tradition states that sailors gave the region the name the "subtropical high" because ships relying on wind power stalled; fearful of running out of food and water, sailors threw their horses and cattle overboard to save on provisions.
Major deserts of the world, such as the Sahara and Great Australian Desert, lie under the high pressure of the horse latitudes. The region is also known as the Calms of Cancer in the northern hemisphere and the Calms of Capricorn in the southern hemisphere.
All of these seem great, but in response to the first reply from pacorz, the horse latitudes are not the same as the doldrums. The doldrums are within a few degrees of the equator where the northern and southern trade winds converge and form the ITCZ (intertropical convergence zone).
The horse latitudes are two high-pressure belts characterized by low winds, at about 30 degrees north and south of the equator. Dreaded by early sailors, these areas have undependable winds with long periods of calm. In the Northern Hemisphere, particularly near Bermuda, sailing ships carrying horses from Spain to the New World (the Americas) were often stalled. When water supplies ran low, these animals were the first to go without water. Dying from thirst or tossed overboard, the animals were sacrificed to conserve water for the men. Explorers and sailors reported that the seas were "strewn with bodies of horses." This is one explanation of how these areas came to be called the horse latitudes.
The term might also be rooted in complaints by sailors who were paid in advance and received no overtime when the ships slowly proceeded through the windless regions. During this time, they were said to be "working off a dead horse."
Sources: Encyclopedic Dictionary of Science, p. 121; Rovin, Jeff. Laws of Order, p. 87; Tufty, Barbara. 1001 Questions Answered About Hurricanes, Tornadoes, and Other Natural Air Disasters, pp. 189-90.