The term "horse" has multiple nautical uses even if nothing can or wants to connect properly now.
Iron bar on deck
An iron bar on deck that was used as a traveler for a sail's sheet
Footrope for balance
The footrope that sailors used to stand on and balance themselves when working on a sail out on a yard
Horse marine
A marine mounted on horseback or a cavalryman doing duty on shipboard
A person out of their proper or natural place
The first month at sea
The first month at sea was called "dead horse time"
It was traditional to haul the effigy of a horse up the mast and set fire to it at the end of the first month. The burning stallion would then be hurled into the sea
White horses
A nautical term for what landlubbers commonly call white caps
Sailors in polar seas care about them because they can conceal smaller icebergs, also known as “growlers”
Horse at Sea and Sky
White crests of foam on the tops of waves are called “horses” and a type of cirrus cloud are known as “mare’s tales.”
Horse and Food
When the salted beef that was fed to the crew was really tough, the sailors would call it “salt horse.” It’s possible that there may indeed have been some ships that fed horsemeat to their crews, but food historians believe that was uncommon.
Horse on maps
The “horse latitudes” are another phrase for the doldrums, the regions around 30° North and 30° South, in which there can be periods of long calms at sea. Some declare, dubiously, that the derivation for this phrase is that ships once threw horses overboard at these latitudes, because the animals had died from the lack of freshwater or even that the sailors were forced to sacrifice the horses to reduce the ship’s weight. Some also contend that somewhere in the horse latitudes or the dead-horse festival is the derivation for the expression “flog a dead horse.”