Complex sentence: Poco Pine was shown about five times as a weanling, winning all his halter classes he was shown in, but Curtner felt that he was slow to develop and didn't show him as a yearling or as a two-year-old.
Split into simpler sentences: Poco Pine was shown about five times as a weanling, winning all the halter classes he was shown in. Curtner, however, felt that he was slow to develop and therefore didn't show him as a yearling or as a two-year-old.