i have found a way to mod horse armor, ill try to explai how it work
<Conditional Code="C"> (the code is "C" for every part)
<Model Index="-">4_RIDING/R0001_HORSE/ARMOR/ARMOR/RH_01_ARMOR_0008_H.PAC</Model> (its a code from the costume u want, not from original item, like a armor mod)
<Model Index="C">4_RIDING/R0001_HORSE/ARMOR/ARMOR/RH_01_ARMOR_0001_H.PAC</Model> (its the original horse armor that u want mod)
</Conditional>
the horse armor is composed by 4 parts, armor, helmet, saddle and stirrup, the above code has 2 "armors" in a row, "armor/armor", all you need to other parts is change the second "armor" for the part that you want, example, armor/saddle or armor/stirrup (except the helmet that has the code "hel")
(that red white horse... next step, how to change your horse color)