When Fred gives that whistle he will unhorse any rider who is on his back.
"Raiding with Morgan"
Byron A. Dunn
They deal such blows upon their shields that, beside the wounded, they unhorse more than five hundred of them.
"Four Arthurian Romances "Erec et Enide", "Cliges", "Yvain", and "Lancelot""
Chretien DeTroyes
Howbeit the Hallers had many horses; and as a lad he had been wont to ride with or without a saddle, and was a rider whom none could unhorse, even in the jousting-ring.
"The Complete Historical Romances of Georg Ebers"
Georg Ebers