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Suppose a ranger puts barding on their animal companion to boost its AC. Does wearing the armor cause this creature to have disadvantage on its attacks, ability checks, and saving throws? Or does the animal companion ignore the penalties somehow?
According to the Armor rules in the PHB, characters who wear armor without proficiency are penalized on their rolls:
If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or Attack roll that involves Strength or Dexterity, and you can’t cast Spells.
However, the PHB rules for Barding do not mention proficiency or these penalties.
Barding is armor designed to protect an animal’s head, neck, chest, and body. Any type of armor shown on the Armor table can be purchased as barding.