Appreciate the suggestions, but tbh I don't really want to change any of the designs of aa0905766k's civs. I think you're kinda right, about the uu redundancy, but maybe it's supposed to represent the huge amount of horses Scythia had available to trade?

The %buff per horse is an interesting idea, I might add it to StS eventually.

Don't the extra horses from the Kurgan make the lack of a horse req for the UU kind of redundant? Maybe slightly increase their combat strength or movement speed instead? Ooh, how about a 1% combat strength buff for each horse in your empire? To play off of the extra horses. Entirely up to you, of course.