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

Adelung:

http://lexika.digitale-sammlungen.de/adelung//lemma/bsb00009132_3_0_649

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.