A flemish horse is a footrope

Footrope

Each yard on a square rigged sailing ship is equipped with a footrope for sailors to stand on while setting or stowing the sails.Formerly, the footrope was the rope sewn along the lower edge of a square sail, and the rope below the yards was called the horse or Flemish horse...

 on a square rigged sailing ship that is found at the extreme outer end of the yard. The main footrope runs along the whole length of the yard, but because of its length the angle upwards to where it is attached is quite shallow, and thus it is too high to stand on for some distance inwards. Sailors on this part of the yard stand on the flemish horse instead, which being shorter hangs down more and hence is low enough to stand on.

The flemish horse, being at the outer end of the yard, often made of thinner rope and attached only at its ends, is somewhat unstable compared with the main footrope. This, together with the requirement to step off the main footrope in order to get onto it means that a place on the flemish horse is generally reserved for the more experienced sailors working on a yard.

The origin of the term is not clear. There is some evidence that at one time all footropes were known as "horses" (e.g., in German, the footrope is to this day called "Fußpferd", i.e. "foot horse"). It has been suggested that Flemish horses (i.e. animals from Flanders

Flanders

Flanders is the community of the Flemings but also one of the institutions in Belgium, and a geographical region located in parts of present-day Belgium, France and the Netherlands. "Flanders" can also refer to the northern part of Belgium that contains Brussels, Bruges, Ghent and Antwerp...

 in Belgium

Belgium

Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

) were regarded as being more unruly than most, and hence the unstable outer "horse" acquired this name.