I don't know how to best attach bone armor; in Warcraft, there's usually just what's left of the mane hanging down. There's also Arthas's horse, but it's "meaty"... I don't think making a "meaty" upgrade would look good. 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 much.