Complex sentence: No one knows just how Messenger arrived in the States, or who brought him here, but he was not only a great sire of thoroughbreds, he was the founding father of the harness breed, or standardbred.
Split into simpler sentences: No one knows just how Messenger arrived in the United States, or who brought him here. He was not only a great sire of thoroughbreds, he was the founding father of the harness breed, or standardbred.