minecraft-src/assets/minecraft/equipment/iron.json

{
  "layers": {
    "horse_body": [
      {
        "texture": "minecraft:iron"
      }
    ],
    "humanoid": [
      {
        "texture": "minecraft:iron"
      }
    ],
    "humanoid_leggings": [
      {
        "texture": "minecraft:iron"
      }
    ]
  }
}