Sorry, missed this one out first time around. The Sundered Eagle has them as Faeries; they enact a customary role by fulfilling stories. If you wanted them to be Magic Realm, I would design them as Magic Humans --- because they can speak and have hands, and having the legs of a horse falls within the Monstrous Appearance Flaw in my opinion. Magic centaurs would have no interaction with humans; there might be stories of their existence, but they would not rely on humans for vitality, so encounters would be unusual.
Mark