The obviously and objectively best typing for this horse is clearly Steel/Steel. Now, you may be thinking, how the fuck can a Pokémon be the same type twice? Well, put simply.... uh.... shut up, it just works. Now allow me to explain why Steel/Steel is at minimum 10x better than all typings ever to exist.
1. It’s unique and unexpected
2. Who doesn’t love the taste of fresh copper?
3. Two times immune to poison. Actually, I have no idea what that means but it sounds special.
4. Horses LOVE steel (trust me on this one).
5. 4x steel STAB? I think? Steel/Steel sounds pretty STABby to me but I’m not sure.
Now, you may be thinking, these are still terrible reasons. You’d be right, but also I don’t care. Have a nice day.