guess it might not be useful but every single type of material that makes armor has a horse armor item that matches that said material. and maybe it could have health or whatever you give the horse. unless his idea is preventative for a reason or such, please let us know
I like this suggestion, but this isn't enough in my opinion, it partially addresses the long standing issue with Nether Fortresses if this is intended to be ultra rare loot found in them. A horse armour that is immune to lava but otherwise provides the same benefits as diamond armour.
We could do with larger Nether Fortress generation, with the additional chambers having new hostile mobs in them, like Demonic Knights, or goblins in full body armour and with enchanted swords, if they won't add goblins in the underground sections of the Overworld, the Nether would be the next best thing here. Obviously what we're going for here is adding in challenging mini bosses, that can do a lot of damage to you and even kill you in two or three hits (5 to 6 hits if you're wearing armour, but being ganged up on is still a problem) if you're not careful, but with this it would feel like the Netherite Horse armour and any other new rare item was truly earned once found.
Netherite Horse armour found at a low probability inside chests inside these new Nether Fortress chambers would be wonderful in my opinion,
also it would be even sweeter if these generated in the core of the structure, or attempted to, forcing players to take a risk by going through the maze if you will if they want the new loot item.