He was mounted on the horse the marshal had given him, as the other was not yet accustomed to stand fire.

The noble animal, as if delighted to be on level ground again, and excited by the roar of battle, carried him along at the top of its speed without any need of urging.

Fergus knew that on the heights behind the king and Keith would be anxiously watching him, for the peril of the cavalry was great; and the concussion of the guns was now causing the fog to lift rapidly and, as he rode, he could dimly make out dark masses of men all along the rising ground behind Sulowitz, and knew that the Austrian cavalry might, at any moment, sweep down on the Prussians.
He was drawing abreast of Lobositz, when suddenly a squadron of cavalry dashed out from the village.

Their object was evidently to cut him off, and prevent any message that he might bear reaching the Prussian cavalry, which were now halted half a mile ahead.
Their officers were endeavouring to reform them from the confusion into which they had fallen, from the speed at which they had ridden and the heavy losses they had sustained.
He saw, at once, that the Austrians would cross his line, and reined in his horse to allow Karl to come up to him.

Had not the trooper been exceptionally well mounted, he would have been left far behind.