Armor of Mount Protection (5e Equipment)
From D&D Wiki
Armor (any armor), rare (requires attunement)
This armor is adorned with various inscriptions of mounted humanoids and various other small signs for protection. You have a +1 bonus to your AC while you wear this armor.
Shared Protection. While worn by you, your mount gains a protective aura around them. Your mount's AC is equal to your AC while you wear this piece of armor unless their AC is higher.
Rejuvenating Shield. When you start your turn on a mount, the mount gains 10 temporary hit points until the start of your next turn.