Well if a fight with a bunch of duck sized horses goes bad, you could easily run away (briskly walk even). If a duck is two hands high vs a horse which might be 16 hands (for a thoroughbred say), then presumably a duck sized horse will run 1/8th of 48 kph (the upper average galloping speed of a full sized horse) or 6kph (a bit under 4 mph).
Of course you must consider that when fighting horses you are fighting somewhat intelligent beasts, albeit little ones, however when fighting a horse sized duck you are really still only dealing with a big bird brain.