The earliest origins of horse armor lay with the Noldor whose unsaddled riders in the first age had started to protect their beloved steeds with light armor.Such traditions later lived on among the Galadhrim who had inherited some of the noldo chivalric spirit from Eregion.
"Carabarfonath" (S.:"Cataphracts"), heavy elite cavalry primarily used for charges to break through opposing heavy cavalry and infantry formation, had later been fielded by the Númenóreans and some Northmen, but had widely come out of use after the Great Northern Wars.The Knights of Dol Amroth were a last remnant if this older military tradition.In Rohan only few knights, the Eorlsons and the Cingesweardas, used a much trimmed down version of the Horse armor.
In the late third Age the Easterlings of Sauron's Golden Army used heavily armed horses and riders, the Lohûria or Lôke-Innas Rim and Dragon-Knights.