Minecraft wiki entry for Development_versions, associated text: The horse rendering and animations are now fully data-driven
This work supports all horse variants (horse, mule, donkey, zombie horse, skeleton horse) and all horse versions including:
v1 (engine version 1.2.5 and earlier)
v2 (engine version 1.2.6 to 1.17.0)
v3 (engine version 1.17.10 and onward). This will be a cleaner version of the v2 model, with less confusing bone naming