Entity Textures

At the moment mojang has not added anything about this, but currently we can add new variants of mobs since 1.21.5, limited but possible through datapacks

📦datapack.zip
  └── 📁data
  │     └── 📁earth_variations
  │         ├── 📁chicken_variant
  │         │    ├── 📑gold_crested.json
  │         │    └── 📑skewbald.json
  │         ├── 📁pig_variant
  │         │    └── 📑pale.json
  │         ├── 📁cow_variant
  │         │    └── 📑<variant>.json
  │         ├── 📁frog_variant
  │         ├── 📁wolf_variant
  │         ├── 📁cat_variant
  │         └── 📁tags
  │              └── 📁worldgen
  │                   └── 📁biome
  │                        └── 📑spawns_pale_variant_pigs.json
  ├── 📝pack.mcmeta
  └── 🎨pack.png
 
📦resource_pack.zip
  ├── 📁assets
  │     └── 📁minecraft
  │          └── 📁textures
  │               └── 📁entity
  │                    ├── 📁chicken
  │                    │    ├── 🎨gold_crested.png
  │                    │    └── 🎨skewbald.png
  │                    └── 📁pig
  │                        └── 🎨pale.png
  ├── 📝pack.mcmeta
  └── 🎨pack.png

gold_crested.json

{
  "asset_id": "minecraft:entity/chicken/gold_crested",
  "model": "normal",
  "spawn_conditions": [
    {
      "condition": {
        "type": "minecraft:biome",
        "biomes": "minecraft:plains"
      },
      "priority": 5
    }
  ]
}

pale.json

{
  "asset_id": "minecraft:entity/pig/pale",
  "model": "normal",
  "spawn_conditions": [
    {
      "condition": {
        "type": "minecraft:biome",
        "biomes": "#earth_variations:spawns_pale_variant_pigs"
      },
      "priority": 10
    }
  ]
}

spawns_pale_variant_pigs.json

{
  "values": [
    "minecraft:snowy_taiga",
    "minecraft:pale_garden"
  ]
}

Example Pack here