In Minecraft 1.21.4, horse-esque mobs (that is, subclasses of AbstractHorseEntity) which do not allow horse armor to be equipped will not show the horse armor slot. In Minecraft snapshot 25w03a, this slot is shown, although no item can be placed into it, and clicking the horse armor onto the mob itself has no effect.

A fuller list of affected entities:

• Donkeys
• Mules
• Llamas
• Trader llamas
• Camels
• Skeleton horses
• Zombie horses