Grimm (link at the bottom) gives two different meanings:
1) a reserve horse which is "at hand"
2) the draughthorses at the right side of the drawbar, which are led with the hand
gives the same meaning. As only Handpferde are mentioned, without other horses, waggons etc., I suspect the Grenadier-Bataillon von Knebel was ordered to guard the assembled officers' reserve horses.