You are trying to give him a defined animal personality. As such, I think InAn should serve your purposes. The personality you are describing is readily observable in mythic europe: warhorses behave like that. Go ahead and do it. it is certainly a cool item Ever thought about challenging Hunger for his starring role in the apocalypse?
If your lab has a Warping Score (see Covenants), it has the chance of infusing any spells or enchantments you invent in the lab with a Personality Trait. However, it seems that you have already made the item, so this might not be a solution.
I would use MuAn - you are altering the corpse of a horse to express a personality, which is something it cannot normally possess because it is dead. I'd look at the MuMe guidelines for a suitable base level.