Under the right conditions, a boat traveling over the ocean or down a river can get badly damaged from a special kind of storm. This storm comes from inside the boat, when its cargo of horses gets a little too restless – in a very coordinated way.
The question of how to get a horse from one place to another generally resolves itself. It’s a horse. Jump on it and go. There have been times in history, however, when large numbers of horses had to be brought from one place to another. At these times the fastest and cheapest way to transport horses was the fastest and cheapest way to transport anything else: by boat.
Moving around living animals was a much tougher challenge than moving around inanimate cargo. Horses, with their weight and skittishness, posed special dangers to any boat that carried them. They could not be crammed, untethered, into the cargo hold, so each horse was led in and tied to the wall, usually in a make-shift stall. Horses lined the walls of the vessel. As long as nothing moved, that wouldn’t be a problem. But if, for example, another boat went by, and its wake made one side of the ship dip, things got messy. The horses on one side of the ship startled and quickly stepped back. Often, great numbers of them would do it at the same time. The motion caused by fifty half-ton animals stepping back would cause one side of the boat to roll. The sudden movement startled the horses on the other side of the ship. They would also step back, causing the ship to roll the other way.
From there it was like a pendulum getting a push at just the right time. Each collective motion of the horses would push the ship down on one side, and when the other side dipped, the collective motion of those horses would cause the other side to dip. The ship would roll back and forth as if it were caught in a storm. These “horse storms” didn’t ever actually capsize a ship, but they could badly damage one in dock.