Context

An 11th level PC in a game that I DM is a ranger6/rogue3/warlock2. He noticed that his animal companion, a black panther, could use a little boost to her defenses, as she goes down frequently. This is due to the PC's multiclassing and his (preferred) reckless approach to combat (face first).

Note: the animal companion currently has 38 HP with an AC of 14. The ranger (Beast Conclave) is from Unearthed Arcana: The Ranger, revised.

His character, therefore, inquired about armor options for his panther. The particular, visited blacksmith replied that "for the right prize, anything is possible, but [he'll] need some time to craft this".

Now, post-session, I'm looking into existing options from the books, but I can't find any. I own all the books, except for XGTE and the adventure modules.

Question

Do any of the books contain (magic) items for animal companions?

I'm mostly interested in armor, but going through the pages I'm wondering if any such item exists. I also allow sources from Unearthed Arcana, so within the scope of this question I count UA as "any of the books".

If not, I'll homebrew the armor for the PC and might possibly follow up on this question.

Research / Confusion

As Sdjz points out in a comment, there's a question that hints to a "yes" on my question: Can a Beast Master ranger armor their animal companion with barding?

Barding. Barding is armor designed to protect an animal’s head, neck, chest, and body. Any type of armor shown on the Armor table in this chapter can be purchased as barding. The cost is four times the equivalent armor made for humanoids, and it weighs twice as much.

I'm attempting to read up on barding with the PHB in front of me. The index says "under Armor and Shield" so I went to page 144-145. But I don't see any mention of barding, under this section. Am I missing something here?

Magical Armor +1/+2/+3

The DMG doesn't specifically list e.g. "Plate Armor +2", but rather the following, generic description (DMG, p. 152):

Armor, +1 , +2 , OR +3

Armor (light, medium, or heavy), rare (+1), very rare (+2), or legendary (+3)

You have a bonus to AC while wearing this armor. The bonus is determined by its rarity.

So, for armor to be possibly available as +1, +2, or +3, it simply has to count as light, medium or heavy armor.

Now, armor is available for animals (called barding), in all varieties available to humanoids, just more expensive (see PHB p. 157). Since barding is therefore also categorized as either light, medium or heavy armor, it is consequentially also available in its magical variants. Therefore, items such as "Studded Leather Barding +2" can exist.

How you calculate the price is up to you: you can use the cost the equivalent magical armor for humanoids (see DMG p. 135 for magic item prices by rarity), plus the price difference between unenchanted humanoid and animal armor.

Alternatively, you can multiply the cost of the magical item by four, i.e. how barding prices are calculated for unenchanted armor. I would suggest the former, though, to avoid excessively high magical barding prices.

Note: mounts are not necessarily proficient in armor/barding, and - like humanoids - will suffer disadvantages if they wear armor that they are not proficient in. Thanks @David Coffron for bringing that up, and see this question for details.

Examples for price calculation (I'll be using the upper limit for magic item prices; again, see DMG p. 135 for magic item prices by rarity, and p. 152 for rarities of magical armor):

Option 1:

$ Studded Leather Barding +1 = Humanoid's Studded Leather +1 + (Studded Leather Barding - Humanoid's Studded Leather) = 5000gp + (4*45gp - 45gp) = 5135gp$

... or (again, only in the case of Studded Leather) 135gp more than whatever you would charge for a suit made for humanoids.

More general: take the price for a suit of magical armor made for humanoids, and add 3 times the value of unenchanted barding of the same armor variety. Magical plate armor for animals would, therefore, be $ 3 * 1500gp = 4500gp $ more expensive than the humanoid equivalent.

Option 2:

$ Studded Leather Barding +1 = Humanoid's Studded Leather +1 * 4 = 5000gp * 4 = 20000gp $

This is easier to calculate, but absurdly expensive. I would advise using option 1.

All other items that are logically wearable by an animal

The DMG states the following on page 141:

When a nonhumanoid tries to wear an item, use your discretion as to whether the item functions as intended. A ring placed on a tentacle might work, but a yuan-ti with a snakelike tail instead of legs can't wear boots.

Therefore, your ranger's panther can't use a magical sword (or nonmagical swords, for that matter), but e.g. an Amulet of Health (CON=19, unless already higher) or a Ring of Protection (AC+1) shouldn't be an issue.

Yes, there is at least one option for armoring an animal companion.

As Can a Beast Master ranger armor their animal companion with barding? points out:

Barding is armor designed to protect an animal’s head, neck, chest, and body. Any type of armor shown on the Armor table in this chapter can be purchased as barding. The cost is four times the equivalent armor made for humanoids, and it weighs twice as much.

Since mounts can use armor through barding, it's safe to say an animal companion could do the same.

Source clarification

Part of your confusion could be caused by an unclarity in the index of the PHB. Although the index mentions to look under Shields and Armor (page 144-145) for barding, you should actually look under Mounts and Vehicles on page 155. And as Rubiksmoose points out in a comment:

When it says "see under x" it means to look under x in the index. In this case looking under "armor and shields" in the index you will see an entry for barding listed for p 155.

Side-stepping the main question a bit, and going for a deeper issue... Your companion has its stats wrong.

Revised Ranger pets should be very durable.

Companions have tons of AC, even without barding custom armor:

An animal companion also adds its proficiency bonus to its AC.

They have lots of HP, which increases even when multiclassing:

For each level you gain after 3rd, your animal companion gains an additional hit die and increases its hit points accordingly.

Your panther companion should have 53HP (13 at level 3, 5HP/level after that) and 16AC (12 natural armor + proficiency).

If you want it even tankier, a single extra-level in Ranger gives the beast great saves:

At 7th level, while your companion can see you, it has advantage on all saving throws.

Now, if you pick a pet with 0 CON and play recklessly, yeah, the pet won't be very durable. But you can resurrect it with an 8h ritual, or heal it with hit dice on short rests. You can also pick more defensive pets (Black Bear), play them safely, or get them magical items (with DM fiat) to make them stronger. I refer to other answers for such items.