It can be jarring for city folk to first see country mages. City mages often have city-suited familiars such as cats, dogs, foxes, crows, seagulls, and so on. But the mages of the rolling hills and steep mountains and endless farmland can have any sort of suited animal, and horses are quite common.
Many country mages practice in healing, or weather magic, or perhaps the wild sort of magic that cares for the forests and rivers. It makes sense, when walking the long dirt roads between the villages, for a horse to be a grand companion. Often they are the foals of working animals, but when the destined bond is seen, farmers rarely put up a fight and instead let the foal go in exchange for the occasional magical boon.