minecraft-assets/assets/minecraft/models/equipment/diamond.json

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