You can manually define variants for entities using NBT.

This is useful for mobs that use custom variant systems that Field Guide can't automatically detect, or for overriding auto-generation.

Field Guide automatically detects variants for many vanilla mobs, including Sheep, Horse, Llama, Fox, Parrot, Rabbit, Axolotl, Mooshroom, and Panda.

data/<namespace>/fieldguide/variants/<filename>.json

{
  "entries": [
    {
      "id": "minecraft:wolf",
      "replace": false,
      "variants": [
        {
          "id": "pale",
          "nbt": "{variant: \"minecraft:pale\"}"
        }
      ]
    }
  ]
}

• id: The target entity registry name.
• replace: If set to true, the Field Guide will completely ignore any automatically detected variants and only use the variants provided in this list.
• variants: A list of variant definitions. If replace is true and this list is empty, the entity will be treated as a standard, non-variant entry (hiding all variant UI and subtitles).

If you need a custom NBT-based mob to appear as its own separate entry rather than as a variant of an existing one, use the nbt_entry content type instead.