Complex sentence: He then began winning race after race and was soon being talked about as a serious rival to Buckpasser, in fact at the time Baeza considered Graustark as the best horse he had ever ridden.
Split into simpler sentences: He then began winning race after race and was soon talked about as being a serious rival to Buckpasser. At the time, Baeza considered Graustark to be the best horse he had ever ridden.