Don't you think there's a kind of "hole" at the horse's neck? It's physiologically correct, but visually it's a bit... so much fabric/clothing, and then a completely bare spot...

I don't know how to best attach bone armor. In Warcraft, there's some of the mane hanging down. There's also Arthes' horse, but it's "meaty"... I don't think it makes sense to make a "meaty" upgrade. It's better to either do nothing or make it look like a horse from Skyrim, but there's no point in that - a glowing horse should have corresponding abilities, and I don't want to overload the unit with abilities.

Of course, you could wrap the spine with straps in a couple of places and attach armor plates to these straps, but... ideally, the armor should be attached to the head and extend to the saddle... I'm too lazy to think about it, make it, and connect all of this. I also need to bend the horns differently for different horses, but the neck doesn't bother me that much.