When summoning an entity minecraft:wolf with {CanPickUpLoot:1b}, the wolf is able to pick up any horse armor and have it render on the wolf. The horse armor is also indestructible. Putting any wolf armor on any other entity has no function.

Can be replicated by the commands:

/summon minecraft:wolf ~ ~ ~ {CanPickUpLoot:1b}, then give the wolf horse armor.

or

/item replace entity [desired minecraft:wolf] armor.body with [minecraft:diamond_horse_armor, golden_horse_armor, iron_horse_armor, leather_horse_armor]

Link to video: Wolf Bug