There are two ways to make horse armor in Minecraft 1. Crafting. 2. Finding.
Minecraft Horse Armor - Minecraft Guides
Minecraft: How To Dye Armor
Jimmy Solidarity coded — Crafter - Automated redstone crafting
Horse Armor Crafter SpigotMC - High Performance Minecraft
Craftable Horse Armour and Saddle Mod for Minecraft 1.9/1.8.9/1.7.10
Iron Horse Armor - Wiki Guide
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Craftable Horse Armor - Minecraft Data Pack
Horse Armor – Minecraft Wiki