Do you mean dark wyvern? Cause dark charger has 72 armor and 43 speed... Anyway, IMO for horse archers are better much faste mounts, the enemy cannot get them. I gave Tamaris white tiger and shes like Lewis Hamilton on meth, doing crazy circles round the battlefield
As to your bow, was it a default bow or some with adjective? Imprpved weapons can have higher requirements but giving a thing to a HG removes the modifier, thus possibly decreasing required power draw (great for repairng damged stuff, just buy cheap damaged equipment amd restore it to the default)