16321 [William Marshal] told the bowmen to make sure to
spread themselves out in a long line,
so that, when the French arrived,
16324 their horses would be killed under them.
The Marshal then asked for
two hundred soldiers and ordered them
to be ready to kill
16328 their own horses with their knives,
so as to be able to take shelter behind them,
if necessary, in an emergency.
All those who listened to the earl
16332 displayed their joy
and disported themselves as merrily
as if they were at a tournament.
William is telling his troops that they are in for a real fight. They will be killing horses instead of taking them as prizes, and they'll even slaughter their own if they need to.
The reaction? Joy.
Source for the passage: De Re Militari.
Source for the photo: A Polish reenactment event described here.